Heat butter and olive oil in a pan over medium heat and add sliced onions. Cook slowly for 12 to 15 minutes, stirring occasionally, until soft and golden brown.
In a small saucepan, combine beef broth, Worcestershire sauce, garlic powder, onion powder, and black pepper. Simmer for 10 minutes, then keep warm.
Preheat oven to 350°F (175°C). Slice slider buns in half, keeping them connected if possible, and place the bottom halves in a baking dish.
Layer roast beef, caramelized onions, and provolone cheese on the bottom buns, then cover with the top buns.
Cover loosely with foil and bake for 15 minutes. Remove foil and bake an additional 5 minutes until tops are golden and cheese has melted.
Optionally brush tops with melted butter. Sprinkle with chopped parsley and crispy fried onions. Serve warm with au jus for dipping.