Take the onions, cut them in slices, and put them in a stewpan, with the butter. Set it in a stove to simmer for an hour, covered up close; take the head, and with a knife and fork pick all the fins you can get off the fish. Put this in a dish, dredge it well with flour, and let it stand. Take all the bones of the head and the remainder, and boil them on the fire for an hour, with an English pint of water. Strain off the liquor through a sieve, and put it to your onions; take the parsley, well washed and picked clean; chop it as fine as possible; put it in the soup; let it just boil, otherwise it will make it yellow. Add the cayenne pepper, anchovy, soy, ketchup, and vinegar. Then put the fish that has been set aside on the plate into the stewpan to the soup, and let it simmer for ten minutes. If not thick enough add a small piece of butter rolled in flour.