Preheat oven to 350°F. In a large skillet over medium-high heat, melt 2 tablespoons butter. Add onion and thyme sprigs, and season with salt and pepper. Cook until caramelized, about 15 minutes. Remove thyme sprigs.
Place bottom halves of slider buns on a large baking sheet. 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. Bake until cheese is melted and sliders are warmed through, about 10 to 15 minutes.
Meanwhile, make au jus: In the same skillet, melt remaining 1 tablespoon butter over medium heat. Add minced garlic and cook until fragrant, about 1 minute. Add beef broth, Worcestershire sauce, and thyme. Season with salt and pepper. Simmer until slightly reduced, about 10 minutes.
Serve sliders with au jus for dipping. Enjoy your delicious game day treat!