Purchase six veal or lamb kidneys. Remove the outer skin, then cut the kidneys into round slices about one-third an inch thick. Let the slices stand covered with cold, salted water about half an hour, then drain and dry on a cloth. Melt three table-spoonfuls of butter in a small saucepan; in it cook a slice of onion and two of carrot; when the vegetables are delicately browned, remove them and put in the pieces of kidney and stir constantly while cooking one minute, then add two tablespoonfuls of flour and stir while cooking one minute, or until the flour is blended with the butter; then add one cup of highly-seasoned brown stock and stir constantly until the sauce is brought quickly to the boiling point. Season with salt and pepper. Lemon juice, Worcestershire sauce, wine or chopped parsley are often used to flavor the dish. Fresh mushrooms are often cooked with the kidneys. Serve on buttered toast.