Robert Sauce

Slice two onions and fry till pale brown. Pour over them as much brown gravy as will cover them. Add 1 tablespoon mustard, 1 teaspoon salt, ¼ tea spoon pepper to simmer gently, adding gravy if necessary till onions are tender. Rub thro' seive. Mix with ¼ pint more gravy. Boil up and serve.