Tear off 4 (18- x 12-inch) sheets of aluminum foil; lightly grease one side of each sheet.
Place 8 meatballs in center on lightly greased side of each foil sheet. Top meatballs evenly with onion and bell peppers. Top each with 1/4 cup marinara sauce.
Bring together 2 sides of each foil sheet over ingredients, and double fold with about 1-inch-wide folds. Double fold each end to form a packet, leaving room for heat circulation inside packet. Place packets on a baking sheet.
Bake at 450° for 30 minutes. Carefully open packets to prevent burns from hot steam. Sprinkle each with 1 tablespoon cheese before serving.