Heat the olive oil in a large saucepan or frying pan, add the onions and cook over a low heat until they are very soft. This will take at least 40 minutes – the onions must eventually disappear into the tomato sauce. Some 5 minutes before the end of cooking, add the garlic.

Now add the tomatoes and stir to break them up. Season with sea salt and black pepper. Cook slowly, stirring occasionally, for at least 1½ hours. When the sauce is ready, it will be dark red and extremely thick, with no juice at all, and the oil will have come to the surface. Serve hot.