Preheat oven to 350°F. Lightly grease a 9x13 baking dish.
Assemble sliders: Slice Hawaiian rolls in half horizontally (keep them connected).
Layer 4 slices cheddar cheese on the bottom half.
Add the roast beef on top of the cheddar.
Finish with 4 slices Muenster cheese, then place the top half of rolls back on.
Transfer to the baking dish.
Make butter topping: In a small bowl, whisk together melted butter, 1 teaspoon au jus powder, Worcestershire, sesame seeds, dried minced onions, onion powder, salt, and pepper.
Top sliders: Pour the butter mixture evenly over the tops of the rolls.
Bake: Cover with foil and bake for 15 minutes. Remove foil and bake for another 15 minutes, until tops are golden brown and cheese is melted.
Make dipping sauce: In a saucepan, whisk the remaining au jus powder with 2 cups of water. Bring to a boil, then reduce to a simmer until slightly thickened.
Serve & enjoy: Slice sliders, serve warm with dipping sauce on the side, and watch them disappear!