Preheat the oven to 400℉. Line a baking tray with foil and lightly spray with olive oil cooking spray.
Add the ground beef, egg, breadcrumbs, Parmesan, Italian seasoning, salt, and black pepper to a large bowl. Use your hands to combine. Divide the meat mixture into 12 equal portions and roll each into a ball. Arrange the meatballs 1 to 2 inches apart on the prepared baking tray.
Bake the meatballs until they’re fully cooked, about 12 minutes.
ASSEMBLE THE SLIDERS:
Preheat the oven to 350℉. Line a large baking tray or 9 by 13-inch casserole dish with foil and spray the bottom with cooking spray.
As 1 unit, slice the rolls in half horizontally. You can do this if you buy slider buns that are all stuck together. Place the bottom half of the rolls on the prepared tray. If you buy buns that are loose, just put the bottom buns on the tray.
Spread ¾ cup of marinara sauce on the rolls, and then place the meatballs on top (1 per roll). Spoon the remaining marinara on top. Arrange the sliced cheese on top of the sauce. Place the top half of rolls on the cheese.
In a small bowl, mix together the olive oil, melted butter, Italian seasoning, and garlic powder. Brush this on top of the rolls.
Cover the rolls with foil and bake until they’re warm throughout and the cheese is melted, about 20 minutes. If needed, broil after baking to brown the tops. Serve warm.