01 - Preheat oven to 350°F. Line a baking sheet with parchment paper.
02 - In a medium saucepan, melt butter over medium heat. Add sliced onions and cook for 6-8 minutes until soft and golden. Remove from heat and set aside.
03 - In a separate saucepan, combine beef broth, Worcestershire sauce, soy sauce, garlic powder, onion powder, salt, and pepper. Bring to a simmer over medium-low heat. Once hot, add sliced roast beef to warm through for 2-3 minutes. Remove beef with tongs, reserving broth for dipping.
04 - Slice slider rolls in half horizontally. Place bottom halves on prepared baking sheet. Spread with mayonnaise and horseradish sauce, if using.
05 - Layer warm roast beef evenly over buns. Top with sautéed onions, then add a slice of provolone cheese to each slider. Cover with bun tops.
06 - Brush the tops of the sliders lightly with melted butter, if desired.
07 - Bake for 10-12 minutes, until cheese is melted and rolls are slightly toasted.
08 - Serve sliders warm with hot au jus in ramekins for dipping. Garnish with fresh parsley if desired.