Skin, wash and wipe the kidneys, cut in thin, round slices, and season with salt and pepper. Put one tablespoonful of butter and half a tablespoonful of flour in the frying-pan, and when hot, put in the kidneys. Stir two minutes, then add half a cupful of stock or water. When the dish boils up, add half a tablespoonful of lemon juice. Serve with a garnish of points of toast.