Soak lambs kidneys one hour in lukewarm water. Drain, clean, cut in slices, season with salt and pepper, dredge with flour, and sauté in butter. Fry one sliced onion and one-half shallot, finely chopped, in three tablespoons butter until yellow; add three tablespoons flour and one and one-fourth cups Brown Stock. Cook five minutes, strain, and add one-half cup mushroom caps peeled and cut in quarters; season with salt and pepper, add kidneys, and serve as soon as heated. White wine may be added if desired.