Beef, calf or lamb kidneys may be used. Be sure they are very fresh. Remove the fat and white center, then soak them for one hour in salted water. Cut them in slices one half inch thick, cover the slices with flour, and saute them for five minutes in the butter. Add to the frying-pan the onion and one half cupful of water, and simmer for ten minutes, not longer. The kidneys will be tough and hard if cooked too long. Just before serving, add the sherry; salt and pepper to taste. One tablespoon of Worcestershire sauce may be used instead of the sherry.