Prep
Preheat oven to 350°F (175°C).
Lightly grease a 9x13-inch baking dish.
Slice slider rolls in half horizontally, keeping them connected.
Assemble Sliders
Place bottom half of rolls in baking dish.
Layer half of the provolone cheese evenly.
Add roast beef in an even layer.
Add caramelized onions if using.
Top with remaining cheese.
Place top half of rolls over the filling.
Add Butter Topping
In a small bowl, mix melted butter, Worcestershire sauce, garlic powder, onion powder, and parsley.
Brush or pour evenly over the tops of the rolls.
Bake
Cover loosely with foil.
Bake for 15 minutes.
Remove foil and bake an additional 5–8 minutes, until tops are golden brown and cheese is melted.
Prepare Au Jus
While sliders bake, combine au jus mix and water in a saucepan.
Bring to a simmer and cook for 3–5 minutes until slightly thickened.
Serve warm for dipping.
Serve
Slice into individual sliders.
Serve immediately with warm au jus.