Take some mackerel, one for each person; clean them well, cut them in two and open flat; they should then be dried, rolled in a little flour, and sprinkled with salt and pepper. Mix a good-sized piece of butter in an oval fireproof dish; when it has reached the foaming stage put in your mackerel flat with the skin uppermost, cook quickly a few minutes, turn them and cook more slowly for a quarter of an hour with a lid on. Serve in the dish with a little lemon juice squeezed on at the last minute. Should the butter have become black, remove it and melt a little more. Whiting, soles and herrings can be treated in the same way.