If you’re craving a rich, savory bite that’s ridiculously satisfying, you’re going to love this Buttery Roast Beef Sliders with Caramelized Onions Recipe. These little sliders bake up soft, buttery, and packed with melty cheese, sweet onions, and that perfect garlic butter topping. Trust me, once you try them, they’ll be your go-to for gatherings or cozy nights in!

The image shows a close-up of two stacked sandwiches on a wooden board. Each sandwich has three main layers: the top and bottom are golden brown toasted buns sprinkled with sesame seeds and seasoning, the middle layer is melted light yellow cheese with caramelized onions that are juicy and golden, and there is a layer of thin slices of cooked meat underneath the cheese. Some melted cheese and onions are dripping out from the sides, making the sandwich look rich and full. The background is dark, and the wood texture underneath contrasts with the food. photo taken with an iphone --ar 2:3 --v 7

Jump to:

Why You'll Love This Recipe
Ingredients & Why They Work
Make It Your Way
Step-by-Step: How I Make Buttery Roast Beef Sliders with Caramelized Onions Recipe
Top Tip
How to Serve Buttery Roast Beef Sliders with Caramelized Onions Recipe
Make Ahead and Storage
Frequently Asked Questions:
Final Thoughts
Buttery Roast Beef Sliders with Caramelized Onions Recipe

Why You'll Love This Recipe

I’ve made these buttery roast beef sliders for parties and family dinners, and every single time, they steal the show. The combination of slow-cooked caramelized onions and melt-in-your-mouth cheese does all the heavy lifting, while the garlic butter brings it home perfectly.

Rich and comforting flavors: The slow caramelization of onions brings a natural sweetness that pairs amazingly with savory roast beef.
Perfect for entertaining: These sliders are easy to assemble and bake, making them ideal for feeding a hungry crowd without stress.
Homemade garlic aioli: Creamy and zesty, it adds a fresh touch that lifts every bite.
Warm au jus for dipping: That extra bit of sauce takes these sliders from good to downright irresistible.

Ingredients & Why They Work

Before you get started, make sure to gather all your ingredients. A few key tips on what to pick: I prefer Hawaiian slider buns for their iconic soft, slightly sweet texture—they really soak up all those buttery flavors. For the roast beef, deli thin sliced works best for layering without overwhelming the slider. And don’t skip on the caramelized onions; they’re what make this recipe truly sing.

Flat lay of a cluster of 3 large yellow onions, thinly sliced in a small white ceramic bowl, a small white bowl of unsalted butter cubes, a small white bowl with olive oil, a small white bowl of light olive oil, a clean large whole brown egg with uncracked shell, a small white bowl with Dijon mustard, a single fresh garlic clove, a small white bowl of fresh lemon juice, a small white bowl with kosher salt, a small white bowl with brown sugar, a small white bowl with finely shredded mozzarella cheese, a stack of thinly sliced deli roast beef, a neat pile of sliced provolone cheese, a cluster of 12 Hawaiian slider buns halved horizontally, a small white bowl with Worcestershire sauce, a small white bowl with poppy seeds, a small white bowl with white sesame seeds, a small white bowl with garlic powder, a small white bowl with dried minced onion, a small white bowl of freshly cracked black pepper, all arranged symmetrically on a clean white marble surface, soft natural light, photo taken with an iPhone, professional food photography style, fresh ingredients, white ceramic bowls, no bottles, no duplicates, no utensils, no packaging --ar 2:3 --v 7 --p m7354615311229779997

Unsalted butter: For controlling salt levels and getting that rich, gooey butter sauce.
Olive oil: Adds a subtle fruitiness and helps soften the onions gently.
Yellow onions: Sweet and perfect for slow caramelizing.
Kosher salt: Essential for seasoning and balancing sweetness in the onions and aioli.
Brown sugar: Optional but speeds up and deepens caramelization effectively.
Beef broth: Used for deglazing and in the au jus, adding robust beefy flavor.
Egg yolk: The base for a silky homemade garlic aioli.
Dijon mustard: Adds tang and cuts through the richness.
Garlic: Freshly grated for punchy but balanced flavor.
Lemon juice: Brings brightness to the aioli.
Light olive or neutral oil: Provides a mild base that emulsifies well for the aioli.
Better than Bouillon Beef Base: Intensifies the au jus depth.
Worcestershire sauce: Adds complexity to both the au jus and garlic butter sauce.
Slider buns (preferably Hawaiian): The soft, slightly sweet buns hold everything together perfectly.
Deli roast beef: Thin slices for that tender, layered effect.
Melty cheese slices: Like provolone, Swiss, or muenster for creamy texture.
Shredded mozzarella: Extra cheesiness on top of the sliders.
Poppy seeds and white sesame seeds: Add a lovely crunch and nutty aroma to the garlic butter topping.
Garlic powder and dried minced onion: Boost the savory, aromatic notes in the butter sauce.
Freshly cracked black pepper: For that finishing touch of spice.

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Note: You'll find the complete list of ingredients, along with their exact measurements, in the printable recipe card at the bottom of this post.

Make It Your Way

One of the best things about this Buttery Roast Beef Sliders with Caramelized Onions Recipe is how much you can tailor it to fit your taste and occasion. Whether you want to keep it classic or add some fun twists, these sliders welcome your creativity with open arms.

Vegetarian Variation: Swap out the deli roast beef for hearty grilled portobello mushrooms or marinated seitan, and use vegetable broth instead of beef broth for caramelizing the onions and the au jus sauce. I’ve tried this for a party, and even the meat-lovers didn’t miss the beef!
Cheese Choices: Feel free to experiment with cheeses—smoked gouda or sharp cheddar add a great twist. I love using Swiss for its nuttiness, but melty provolone keeps it super classic and comforting.
Speed It Up: If you’re short on time, use mayo jazzed up with garlic, mustard, and lemon instead of making the aioli from scratch, and skip caramelizing the onions. This shortcut still gives you that crave-worthy flavor in about 30 minutes.
Spicy Kick: Add a swipe of your favorite hot sauce or a sprinkle of red pepper flakes into the garlic butter topping before baking. I like a little heat to contrast the buttery richness!
Seasonal Additions: Toss in some sautéed mushrooms or roast red peppers along with the caramelized onions for an earthy, seasonal boost that impresses every time.

Step-by-Step: How I Make Buttery Roast Beef Sliders with Caramelized Onions Recipe

A woman's hand is holding a sandwich made with a toasted, golden-brown sesame seed bun, dripping with thick brown gravy. Inside the sandwich, there are multiple layers of soft, thin slices of cooked meat and melted cheese with a creamy texture, visible just under the bun. Below the sandwich, a white bowl filled with rich brown gravy has some gravy dripping down its side onto a dark wooden surface. Behind the bowl, several more sandwiches with the same sesame seed buns, meat, and cheese are stacked, showing their melted cheese and crispy textures. The whole scene is set on a white marbled surface with a blurred dark shirt in the background, capturing the warm, messy, and inviting aspect of the dish. photo taken with an iphone --ar 2:3 --v 7

Step 1: Slowly Caramelize Those Onions

Start by melting 2 tablespoons of butter and 1 tablespoon of olive oil in a large skillet over medium-low heat. Add your thinly sliced onions with ½ teaspoon kosher salt and, if you want a little help speeding the process, 1 teaspoon of brown sugar. Patience is key here—let them cook gently for about 45 minutes until they turn deeply golden, soft, and jammy. Towards the end, splash in 2 tablespoons of beef broth to deglaze the pan and scoop up all that beautiful caramelized flavor. Once done, set them aside and get ready for the next step.

Step 2: Whip Up the Creamy Garlic Aioli

In a small bowl, whisk together a room temperature egg yolk, 1 teaspoon Dijon mustard, 1 finely grated garlic clove, and 1 tablespoon fresh lemon juice until smooth. Then, either slowly drizzle in ½ cup of light olive or neutral oil while whisking vigorously or blend gently with an immersion blender until thick and creamy. Season with kosher salt to taste. Don't worry if you prefer to skip the raw egg yolk—just mix mayo with garlic, mustard, lemon juice, and salt for an easy, flavorful alternative.

Step 3: Simmer a Rich Au Jus Sauce

Melt ¼ cup unsalted butter in a medium saucepan over medium heat, then stir in 1 tablespoon Better than Bouillon Beef Base, 2 cups beef broth, and 2 teaspoons Worcestershire sauce. Let this simmer gently for about 20 minutes, until it reduces by half, darkens slightly, and thickens a bit as it cools. Keep it warm so it’s perfect for dipping when your sliders are ready.

Step 4: Toast Those Bottom Buns Just Right

Preheat your oven to 350°F (180°C or Gas Mark 5). Without separating the slider buns, slice them horizontally and arrange the bottom halves on a parchment-lined baking sheet, cut side up. Lightly brush them with melted butter or a touch of your prepared garlic aioli. Pop them in the oven for 5 minutes until they're just golden and slightly crisp—a little crunch helps to hold all that juicy goodness together.

Step 5: Layer on the Roast Beef, Cheese, and Onions

Spread a generous layer of garlic aioli over the toasted bottoms, then pile on 2–3 slices of thinly sliced deli roast beef per slider for hearty, satisfying bites. Top each with a slice of melty cheese of your choice, then sprinkle ½ cup shredded mozzarella across the group for that perfect gooey finish. Finally, spoon on about 2 cups of your luxuriously caramelized onions—trust me, they’re the star that steals the show.

Step 6: Finish with Garlic Butter and Bake

Place the top buns on each slider. In a small bowl, whisk together 6 tablespoons melted unsalted butter, 1 teaspoon Worcestershire sauce, 1 tablespoon poppy seeds, 1 tablespoon white sesame seeds, ½ tablespoon garlic powder, ½ tablespoon dried minced onion, ½ teaspoon kosher salt, and freshly cracked black pepper to taste. Brush this flavorful garlic butter mixture generously over the slider tops. Cover everything loosely with foil and bake for 10 minutes. Then, remove the foil and bake for another 10 minutes until the tops are golden brown and the cheese is gorgeously melted.

Step 7: Serve Warm with Au Jus for Dipping

Carefully slice between each roll to separate the sliders. Serve them warm alongside small bowls of the hot au jus sauce—perfect for dipping each bite into savory bliss. Gather your friends and family because these sliders disappear fast!

Top Tip

Making Buttery Roast Beef Sliders with Caramelized Onions Recipe is truly a joy, and a few insider tips can help you nail that perfect balance of rich, savory flavors and tender textures every time.

Slow and Steady Onions: Patience is key! Low and slow caramelization over about 45 minutes brings out that deep sweetness in your onions, which makes all the difference in flavor.
Layer Your Sliders Carefully: When piling on the roast beef, 2–3 slices per slider creates hearty, satisfying bites—less can feel skimpy, and more might overwhelm the bun.
Butter Brush Magic: Don’t skip the garlic butter brushed on top of the buns before baking. That combination of melted butter, Worcestershire, and seeds gives the sliders a gorgeous golden crust and amazing flavor boost.
Don’t Overcrowd the Pan: When caramelizing onions or toasting buns, keep things spread out for even cooking. Overcrowding can lead to steaming instead of browning.

How to Serve Buttery Roast Beef Sliders with Caramelized Onions Recipe

A tan baking tray holds five slices of light yellow bread with a soft texture. Each slice is spread with a layer of creamy white mayonnaise, which is smooth with small bits of green herbs mixed in. The slices are arranged close together, with three on the bottom and two on the top, placed on a white marbled surface. Photo taken with an iphone --ar 2:3 --v 7

Garnishes

For a fresh pop and extra texture, try adding a sprinkle of chopped fresh parsley or chives just before serving. A few pickled jalapeño slices or pickled red onions can add a pleasant tang and contrast the richness beautifully. If you love a little heat, a drizzle of horseradish sauce or a smear of spicy mustard pairs wonderfully with the savory roast beef and sweet onions.

Side Dishes

These sliders shine alongside crispy French fries or seasoned potato wedges. For a lighter twist, serve with a crisp green salad tossed in a tangy vinaigrette. Roasted vegetables or a creamy coleslaw also add balance and complement the rich flavors. And don’t forget extra small bowls of warm au jus for dipping—that’s part of the magic!

Make Ahead and Storage

Storing Leftovers

Once your sliders have cooled, store any leftovers in an airtight container in the refrigerator for up to 3 days. Keeping them covered helps maintain their moisture and keeps those buttery buns from drying out.

Freezing

If you want to freeze your sliders, it's best to assemble them without the caramelized onions and garlic butter topping. Wrap them tightly in plastic wrap and then foil to prevent freezer burn. They’ll keep well for up to 1 month. When ready to enjoy, thaw them overnight in the fridge before baking with the onions and butter as directed.

Reheating

Reheat your leftovers in the oven at 375°F for about 10 minutes. This method warms everything through without making the buns soggy, keeping the cheese melty and the tops buttery and crisp.

Frequently Asked Questions:

Can I make the caramelized onions faster?

Yes! Adding 1 teaspoon of brown sugar helps speed up the caramelization, reducing the total cooking time to around 35 minutes instead of 45. Just be sure to cook on medium-low heat to avoid burning.

Is it safe to use raw egg yolk in the garlic aioli?

For food safety, make sure to use pasteurized eggs if you’re concerned about raw egg. Alternatively, you can substitute the aioli with quality mayonnaise mixed with garlic, Dijon mustard, lemon juice, and salt for a safe and tasty shortcut.

What kind of cheese works best for these sliders?

Melty cheeses like provolone, Swiss, or muenster are ideal because they melt beautifully and complement the roast beef and caramelized onions. The shredded mozzarella adds a lovely gooey texture on top.

Can I use other types of slider buns?

Absolutely! Hawaiian rolls are preferred for their soft, buttery texture, but any slider buns you have on hand will work well. Just make sure to toast the bottoms before assembling for the best texture.

Final Thoughts

These Buttery Roast Beef Sliders with Caramelized Onions Recipe are one of those dishes that bring people together. Whether you’re feeding a crowd or indulging on a cozy night in, they offer layers of flavor and comforting textures that feel like a warm hug on a plate. With a little patience and love (plus that signature garlic butter topping), you’ll have a crowd-pleaser that satisfies every time. So gather your ingredients, invite some friends, and enjoy every delicious bite!