Including potatoes in pureed vegetable soups is a great way to give them body as well as a subtle background flavor.  That's the secret to this soup's success, too. Fresh spinach and watercress are a sprightly combination that produces a lovely verdant soup with a peppery taste. There's no cream used in the soup itself, though here it's served with a dollop of creme fraiche for extra richness (a little goes a long way).