Select 8 sprigs of the best looking watercress and set aside for a garnish.Wash watercress and discard any tough stems. Chop watercress. Set aside. In a large saucepan melt butter over medium heat.Add leeks and potatoes;cook, stirring constantly, five minutes, until leeks are softened but not brown. Stir in chicken broth and tarragon. Bring to a boil over high heat, stirring constantly. Reduce heat to med.-low; simmer, covered, 20 minutes, stirring occasionally, until potatoes are tender. Stir in watercress. Increase heat to medium; boil 1 minute until watercress is limp and bright green. In blender or processor, purŽe soup in small batches until smooth. Return to saucepan, add milk and seasonings. Refrigerate,covered, until ready to serve. Before serving,heat thoroughly but do not boil. Adjust seasonings. Put into soup bowls and garnish with watercress.

Serves 8.