Skin the kidneys and remove the cores. Soak for 5 min. in cold water. Dry and cut into ¼-in. slices. Finely chop the shallots, heat the fat in a sauté pan and fry them slightly. Then put in the sliced kidney and shake and toss over the heat for about 5 min. Drain off the surplus fat and add the brown sauce, sherry (if used) and salt and pepper. Stir over a gentle heat until thoroughly hot, but take care not to let the mixture boil. Serve hot on toast or fried bread, garnished with watercress.