Fry in a small saucepan three medium-sized onions, chopped fine, in a tablespoonful of butter; stir them as they fry, till they are clear and brownish; add to them half a pint of brown sauce and a wineglass of water or broth to allow for the boiling away; boil twenty minutes and strain; then mix one teaspoonful of vinegar in a cup with a teaspoonful of mustard, which stir into the sauce and it is ready to use.