Heat the olive oil in a frying pan and fry the onion for 5-7 minutes.
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. Serve.