Shred 250 g (9 oz) white of leek and stew gently with 75 g (2½ oz) butter. Moisten with 1¾ litres (3 pt or 7½ U.S. cups) water or White Bouillon. Add 500 g (1 lb 2 oz) potatoes cut into thin slices from potatoes trimmed to the shape of corks and season with 15 g (½ oz) salt if using water. Cook together gently.
Finish at the last moment with 75 g (2½ oz) butter and add dried slices of French bread to the soup when serving.