Wash the kidneys well, remove any skin and cut out the cores with scissors. Soak in tepid salt water for 10–15 min. Wash well in clear tepid water. Cut the kidneys in halves lengthwise. Cut each half into slices about ½ in. thick and toss in seasoned flour. Chop the onion finely, heat the fat and cook it until lightly coloured. Then add the sliced mushrooms and prepared kidney and cook quickly until browned. Pour off any surplus fat and add the stock or gravy, the sherry (if used) and seasoning. Stir until boiling and simmer very gently for ½ hr. Add the chopped parsley, stir well and serve on a hot dish. Garnish with croûtons of fried bread or pastry.