We recommend using the freshest watercress possible — if not at its best, it can give the sauce a slightly bitter taste. (Try hydroponic watercress; it's generally better than conventionally grown supermarket cress.)
Tanis suggests skimming all the fat from the cooking liquid and serving the broth as a first-course soup, garnished with toasted slices of French bread.
Cooks' notes:
• Stuffed and seasoned uncooked chicken can be chilled, loosely covered, up to 1 day.
• Watercress sauce can be made 2 hours ahead and chilled, covered.