Preheat oven to 375 degrees Fahrenheit.
Line a large baking sheet with foil & then place the raw meatballs on it about 1″ apart.
Bake the meatballs for 15 minutes.
Remove the baking sheet, turn the meatballs over & return to the oven to bake an additional 14-17 minutes or until internal temperature reaches 185 degrees F.
While the meatballs are baking, spray your casserole dish with oil or cooking spray.
Place the buns in the prepared dish & line the inside of each bun with a slice of provolone cheese.
Once the meatballs are cooked, place several in each bun in between the cheese. (*note- do not turn off your oven after baking the meatballs)
Spoon the marinara sauce over the meatballs & then top each sandwich with a handful of mozzarella cheese.
Bake in the 375-degree F oven for 10-15 minutes or until the cheese is melted.
*Opt – broil for 2-5 minutes to brown the cheese before serving.
Sprinkle with Parmesan cheese & parsley prior to serving.