Cut mozzarella cheese into 3/4-inch cubes. Store in refrigerator while preparing the meatballs.
In a large bowl, mix sausage, beef, salt, pepper, onion, garlic, Parmesan, breadcrumbs, parsley, Worcestershire sauce and egg. Form meatballs with the mixture. Press a mozzarella cube into the center of each and roll the edges of the meatball around it until it’s covered.
Arrange meatballs in the slow cooker and cover in marinara.