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.

Notes
To make these in the air fryer – cook the meatballs at 390 for 15 minutes. Assemble the sandwiches, then coo for another 3-5 minutes or until cheese is melted.