Peel oranges and remove all white membranes.
Slice onion very thin, place in a bowl and cover with sherry. Cover and marinate for 2 hours. Place lemon juice, oil, honey, pepper and liqueur in a bowl add 2 tablespoons sherry marinade and beat with a wire whisk. Slice oranges very thin and lay over watercress overlapping circles. Beat the dressing and pour over the oranges.