Heat half the olive oil in a frying pan. Fry the onion for 5-7 minutes until soft and translucent.
Add the whole garlic cloves and cook for about one minute. Season with salt and freshly ground black pepper.
Add the chopped tomatoes and the stock. Let the mixture simmer and reduce for about 40 minutes. You may wish to add more stock and let it boil for longer, or add further seasoning, according to taste.
Place the veal escalopes on a board and cover them in cling film. Beat each one with a meat mallet or rolling pin until they are about 1cm/½in thick.
Coat the veal in flour, salt and freshly ground black pepper, then heat the remaining olive oil and sauté the veal for 2-3 minutes on each side.
Add the sauce to the frying pan, then place a slice of mozzarella on each veal escalope, and spoon some of the sauce on top. Simmer for five minutes.