While the duck is roasting, bring the duck or chicken stock up to the boil with the thyme, butter and peeled rind from one lemon. Place the lettuce into the stock and cover with a lid or tin foil.
Boil for 1 minute, then add the lemon juice, chopped watercress and any duck cooking juices ... watercress after reheating the sauce. This recipe is not suitable for freezing.