1 large onion, thinly sliced
2 sprigs, plus 1/4 teaspoon fresh thyme
Kosher salt
Freshly ground black pepper
12 slider buns, halved
1 lb. thinly sliced deli roast beef
12 slices provolone cheese
1/4 tsp. garlic powder
1 tbsp. clove garlic, minced
1 1/2 c. low-sodium beef broth
1 tbsp. Worcestershire sauce
Preheat oven to 350°. In a large skillet over medium-high heat, melt 2 tablespoons butter. Add onion and thyme sprigs and season with salt and pepper. Cook, stirring occasionally, until caramelized, about 15 minutes. Discard thyme. Place bottom halves of slider buns on a large baking sheet and top with roast beef, provolone cheese, caramelized onions, and slider bun tops. Melt 2 tablespoons butter and brush on top of buns. Sprinkle with garlic powder, coarse salt, and parsley and bake until cheese is melty and sliders are warmed through, 10 to 15 minutes. Meanwhile, make au jus: Add remaining 1 tablespoon butter to same skillet and melt over medium heat. Add garlic and cook until fragrant, 1 minute, then add beef broth, Worcestershire, and thyme and season with salt and pepper. Simmer until slightly reduced, 10 minutes. Serve sliders with au jus for dipping.