Cut the eel in 3 or 4 inch lengths, and sprinkle it with salt—the French always use “gros sel,” a very coarse salt, which gives an excellent flavour—and let it stand for a few hours, or overnight. Then wash the pieces, and cook gently for about 10 minutes in a little butter. Slice the onions finely and brown in hot butter. Now put the pieces of eel in a saucepan, add about 3 or 4 pints of water, the onions, the tomato purée, the seasoning, bring to the boil and simmer till the fish is quite tender, which will take about 1 hour. If the potatoes are large, they should be added about ¾ of an hour before serving, if small ½ an hour will be sufficient. In some parts of France, the pieces of eel are left whole, and eaten separately, as in Brittany, or the eel can be removed from the soup, skinned, and rubbed through a sieve, this purée being well mixed into the soup.