1. Preheat the broiler on high.
2. Coat a pan with cooking spray and heat over medium-high heat. Add the meatballs, turning occasionally until they're brown and caramelized all over, about 10 minutes. Add in marinara sauce and turn the heat down to medium-low. Cook for an additional 4 minutes.
3. To make sandwiches, add 3 meatballs to each of the 4 rolls, pouring remaining marinara sauce evenly over the top of each sandwich. Cover with 1/4 of the parmesan and 1/4 of the mozzarella. Wrap the bottom of the sandwich in tin foil to stabilize, then broil, meatball side up, until cheese is melted and bubbly, about 3 minutes.