Preheat the Oven: Set your oven to 350°F and let it heat up while you prepare the sliders.
12 slider buns
Caramelize Onions: In a skillet over medium heat, melt 2 tablespoons of butter. Add sliced onions and thyme sprigs. Season with salt and pepper, stirring occasionally, and cook until caramelized, about 30 minutes. Remove and discard the thyme.
5 tablespoons unsalted butter, 1 large yellow onion, 2 sprigs thyme, Kosher salt, Black pepper
Assemble Sliders: Place the bottom halves of the slider buns on a baking sheet. Layer with roast beef, provolone cheese, and caramelized onions. Add the top buns to close the sliders.
1 pound thinly sliced deli roast beef, 12 slices provolone cheese
Brush and Season: Melt 2 tablespoons of butter in the microwave. Brush it over the tops of the buns, then sprinkle with parsley, garlic powder, and a pinch of salt.
1 teaspoon chopped fresh parsley, 1/4 teaspoon garlic powder
Bake Sliders: Bake in the oven for 10 to 15 minutes, or until the cheese is melted and the sliders are warmed through.
Prepare the Jus: In the same skillet used for the onions, melt the remaining tablespoon of butter over medium heat. Add chopped garlic and cook until fragrant, about 1 minute. Stir in beef broth, Worcestershire sauce, and chopped thyme. Season with salt and pepper. Simmer for about 10 minutes, allowing it to slightly reduce.
1 clove garlic, 1 1/2 cups low-sodium beef broth, 1 tablespoon Worcestershire sauce
Serve: Transfer the sliders to a serving platter. Garnish with extra parsley and serve with the warm au jus for dipping.