Preheat the oven to 350°F (175°C). Lightly grease a baking dish or sheet pan large enough to hold the sliders closely together.

In a skillet over medium heat, melt the butter with the olive oil. Add the sliced onion and cook for 5 to 7 minutes until softened and lightly caramelized.

Stir in the Worcestershire sauce, garlic powder, and a pinch of salt and black pepper. Remove from the heat and set aside.

In a small saucepan, combine the beef broth, soy sauce, Dijon mustard, and onion soup mix. Bring to a gentle simmer, then keep warm over low heat.

Slice the rolls in half horizontally without separating the individual rolls. Place the bottom halves in the prepared baking dish.

Spread mayonnaise and horseradish, if using, over the bottom rolls. Layer on the roast beef, cooked onions, and provolone cheese, then place the top halves of the rolls over the filling.

Brush the tops with melted butter and sprinkle with sesame seeds or poppy seeds, if desired.

Cover loosely with foil and bake for 10 to 12 minutes, until the cheese is melted and the sliders are heated through. Remove the foil and bake for 2 to 3 minutes more to lightly toast the tops.

Serve warm with the prepared beef broth dipping sauce on the side.