Have the kidneys perfectly fresh; split them in half; trim with a sharp knife, very carefully, the sinews and fat that are inside. Now cut the kidneys in small pieces, put in a pan, cover with cold water, stand on a moderate fire and bring almost to a boiling point. Drain this water off, cover with fresh cold water and heat again; do this three times. Do not let it boil, or the kidney will be tough and hard. Put a tablespoonful of butter in a frying pan, stir until brown; add one tablespoonful of flour, one-half pint of boiling water or stock; stir constantly until it boils. Now add one tablespoonful of Worcestershire sauce, one tablespoonful of mushroom catsup, salt and pepper, and the kidney. Stir again until the kidney is thoroughly heated, take from the fire, add four tablespoonsful of sherry, and serve immediately.