Put the beef in a bowl with half the crushed garlic and half the onion, the breadcrumbs and 25 g (1 oz) of the Parmesan. Season and use your hands to thoroughly blend the ingredients together. Shape into small balls, about 2.5 cm (1 inch) in diameter.
Heat half the oil in a large frying pan or sauté pan and fry the meatballs, shaking the pan frequently, for about 10 minutes until browned. Drain.
Add the remaining oil and onion to the pan and fry until softened. Add the wine and let the mixture bubble until the wine has almost evaporated. Stir in the remaining garlic, the tomatoes, sugar, tomato paste and a little seasoning. Bring to the boil and let the mixture bubble until slightly thickened.
Stir in the olives, all but 1 tablespoon of the oregano and the meatballs. Cook gently for a further 5 minutes.
Arrange the mozzarella slices over the top and scatter with the remaining oregano and Parmesan. Season with black pepper and cook under the grill until the cheese starts to melt. Serve in shallow bowls with warmed, crusty bread.