Slice the kidneys, after they have been soaked in cold water; wipe dry and roll in flour. Have ready in a saucepan a little butter in which has been fried a slice of onion. Lay in the kidneys; roll them over and over, coating them with the butter, for two minutes - no more - and pour in a cupful of boiling water or heated stock. Simmer not longer than ten or twelve minutes. Take them up and lay upon a hot dish; add to the gravy a tablespoonful of catsup, a dash of paprica or cayenne, and salt, a small tablespoonful of butter that has been rolled in browned flour, and when it has boiled up, a generous glass of sherry or claret. Pour over the kidneys and serve.