Cream of Watercress Soup with a Toasted Walnut Garnish

This exquisite pale green summer soup is satisfying and sophisticated. It’s thickened with potatoes, not flour, giving the soup a velvety texture, while the toasted nuts add a surprising and delicious finishing touch. Try to use walnut halves or whole nuts rather than the cheaper walnut pieces, which come from the bottom of the sack and are often dusty. It can be served hot or chilled but in either case the flavour is best if the soup is made at least 12 hours before you plan to serve it. F