1. Soak the beans in water, overnight. Bring the same water to a boil, reduce the heat and simmer until the beans are tender,. add more water if necessary. Drain and reserve the cooking liquid.
2. In a skillet brown the onions and peppers in oil until tender. Purée in a blender. Mix with tomato sauce, salt, pepper and oregano. Cook, stirring, for 5 minutes. Add the beans and cook 10 minutes more, adding cooking liquid until desired consistency. Add the sugar and vinegar. Cool and refrigerate, if to be served in a few days. Reheat and correct seasonings.