In the slow cooker combine meatballs, passata, garlic, oregano, basil and sugar. Stir well.
Cook on high for 3-4 hours.
Cut your subs in half and spoon the sauce from the slow cooker onto both sides of the bread. Place 5-6 meatballs onto one side, spoon over some more sauce and top with mozarella cheese slices.
Optionally grill (broil) for 2-3 minutes until the cheese melts.