Chilled Potato-Leek Soup with Fennel and Watercress (Vichyssoise)

Similar in appearance to celery, with a large bulb on the end, fennel has a faint licorice flavor. It is available year-round but is at its peak from October to March. Select creamy-colored bulbs with no browning, topped by crisp stems and feathery green leaves. Cut the stems off about 2 inches from the bulb and use only the bulb, trimming away the base of the core if it is thick and tough. The leaves make an excellent garnish and also can be used for flavoring soups and salads.