Mix the mince and all the ingredients up to and including pepper in a bowl. Knead well until the ingredients form a compact mass. Divide the mixture into 12 portions and, with wet hands, shape into balls. Insert 1 mini-Mozzarella into the centre of each.
Heat the oil in a frying pan. Fry the meatballs in batches over a medium heat for approx. 10 mins. all over. Remove, keep warm. Wipe the cooking fat from the pan, add the passata, season and simmer for approx. 5 mins. Add the meatballs to the sauce, cover and simmer for approx. 5 mins.
Bring the milk and stock to the boil, stir in the polenta, simmer for approx. 2 mins. over a very low heat, stirring constantly to form a thick paste. Stir in the cheese, season. Serve with the meatballs, scatter with basil.