Soak, pare, trim, and slice six kidneys. Sprinkle with salt and pepper, sauté in butter, and remove to a hot dish. Cook one-half tablespoon finely chopped onion in two tablespoons butter until brown; add three tablespoons flour, and pour on slowly one and one-half cups hot stock. Season with salt and pepper, strain, add kidneys, and one tablespoon Madeira wine.