Soak the fish for seven days, changing the water twice a day.
Put it into an enamelled pan and cover with plenty of cold water. Bring very slowly to just below boiling point so that this process takes up to 15 to 20 minutes. Turn off the heat and leave to stand for 20 minutes.
Remove the fish from the water and in its place boil the potatoes in their skins. When they are cooked,