Step 1

In a medium bowl, combine ground beef with egg, bread crumbs, garlic powder, salt, and pepper. Mix with hands or a large spoon until all ingredients are well incorporated.

Step 2

Form mixture into 12 meatballs, taking care not to make them too compact. Place meatballs in the bottom of slow cooker. Pour pasta sauce over meatballs, making sure that each one is completely covered.

Step 3

Cook on LOW for 6 hours or until the meatballs are cooked through. (Check sooner if your slow cooker runs hot, in which case the meatballs may cook faster and the sauce may start to darken around the edges.)

Step 4

To serve, halve each roll lengthwise, toast if desired, and place 3 meatballs inside. Sprinkle with mozzarella cheese. The hot meatballs should cause the cheese to melt, but if not, you may place sandwiches under the oven broiler for a minute to melt cheese completely. Sprinkle with Parmesan cheese, if using, and serve hot.