Heat the oven to 400°F. Remove the bread from the bag. Carefully separate the bread halves with a fork. Place the 2 bread halves, cut-side up, onto a baking sheet.
While the oven is heating, start the sauce and meatballs. Cook in a 2-quart saucepan over low heat for 20 minutes or until the meatballs are heated through. Spoon the meatball mixture onto the bottom bread half. Top with the top bread half. Cut into quarters.
Customize these sandwiches by topping with even more cheese (try Parmesan or provolone), spinach, arugula or chopped fresh basil.