In a large bowl, combine beef, bread crumbs, egg, onion, Parmesan cheese, salt, and Worcestershire sauce.

Using your hands, mix just until combined. Form into one-inch meatballs.

Cook in a large skillet, turning occasionally, until browned on all sides.

Remove meatballs to a slow cooker; top with pasta sauce.

Cover and cook on low setting for about 2 hours; stirring after one hour.

Spoon several meatballs into each bun; top with a spoonful of sauce and some cheese.