Place all the sauce ingredients in the slow cooker and stir to combine
Set the slow cooker to high and cover until your done preparing the meatballs (30 minutes should suffice).
Mix all meatball ingredients excluding the mozzarella in a large bowl. Using your hands, work with the mass as if you were making pizza dough.
Take a golf ball-sized piece of the beef mixture and place a piece of mozzarella in the center
Press the beef around the mozzarella piece until fully enclosed. Repeat until you run out of meat.
Transfer the meatballs into the slow cooker and immerse in the sauce. Cover and continue cooking on high for up to 2 ½ hours