Put onions in a large saucepan and cook over medium heat until the moisture is released and evaporated, about 3-4 minutes. Add butter and olive oil and cook for 6-8 minutes or until onions are golden brown. Add peppers and tomatoes. Cook for 4-5 minutes or until the water from vegetables is evaporated. Add vinegar and salt. Cook 3-4 minutes or until vinegar is absorbed. Add water, cover and cook for 5 minutes or until completely heated through. Serve hot with meat or cold as an appetizer. Serves 4.