Pass tomatoes (or buy tomatoes that have been strained). Heat the butter and oil in a large saucepan and sauté the onions in it. Add the tomatoes and all other ingredients, bring to the boil and reduce over low heat for approx. 60 minutes. Grilled meat can be marinated in the sauce, or the meat can be coated with it while it is roasting. Of course, it can also be used like a normal barbecue sauce.