If using a whole prime rib roast, let it rest until cool enough to slice thin (about 20–30 minutes after resting). Slice roast thinly across the grain into pieces roughly 2–3 mm thick. If using pre-cooked leftover roast, slice thinly. Place slices in a bowl and toss gently with kosher salt, black pepper, garlic powder and 1 tablespoon olive oil to coat evenly.

Warm: Heat a large skillet over medium-high heat. Add a drizzle of oil if pan is dry. Working in batches, quickly sear slices (15–30 seconds per side) just to warm and develop a little color. Remove to a plate and keep warm wrapped loosely with foil while you finish other components.

Caramelized Onions

Cook onions low and slow

Heat a wide skillet over medium heat. Add 1 tablespoon olive oil and 2 tablespoons butter. When butter is melted, add the thinly sliced onions, sprinkle with salt and pepper and the optional brown sugar. Stir to coat.

Reduce heat to medium-low. Cook onions, stirring every 3–5 minutes, allowing them to soften and turn golden. If onions begin to stick or dry out, add 1–2 tablespoons water or beef stock to deglaze the pan and scrape up browned bits. Continue cooking 25–35 minutes until the onions are deep golden-brown and sweet. Remove from heat and keep warm.

Horseradish Cream

Mix sauce

In a small bowl combine 2 tablespoons prepared horseradish, 1/2 cup sour cream, 2 tablespoons mayonnaise, 1 teaspoon Dijon, 1/2 teaspoon lemon juice and 1/2 teaspoon Worcestershire sauce. Season with 1/4 teaspoon salt and 1/4 teaspoon black pepper. Taste and adjust: add more horseradish for heat or more sour cream to mellow. Chill until assembly so flavors meld (at least 15 minutes).

Slider Assembly

Prep buns

Preheat oven to 350°F (175°C) or heat a skillet. Slice slider buns in half. Brush cut sides lightly with softened butter. Place buns, cut side up, on a baking sheet and toast in oven for 4–6 minutes until edges are golden, or toast in a skillet until lightly browned.

Assemble sliders

Spread about 1 teaspoon of horseradish cream on the bottom half of each bun. Add a small handful (about 1–1.5 oz / 30–45 g) of warm prime rib slices folded to fit. Top beef with 1–2 tablespoons caramelized onions and a few leaves of arugula or spinach if using. Finish with a dollop (about 1 teaspoon) of extra horseradish cream on the top bun if desired. Close sliders.

Serve: Arrange sliders on a platter and serve warm. Provide extra horseradish cream and caramelized onions on the side for guests who want more. Leftovers: keep components separately refrigerated for up to 3 days and reheat beef gently to avoid overcooking.