Make a cross hatch pattern cutting through duck skin but not meat. Brush with soy sauce and sprinkle with peppercorns and coriander. Marinate for one hour or overnight.
In a skillet on medium heat, add duck breasts, skin side down. Sauté for 3 minutes or until skin is deep brown. Remove to baking dish and bake, skin side up, for 12 to 14 minutes or until duck is medium rare.
Discard fat from skillet, add sugar and vinegar and cook until sugar carmelises, about 2 minutes. Pour in soy sauce, orange juice and stock and boil until slightly thickened (about 5 minutes). Add Grand Marnier, bring to boil, remove from heat and swirl in butter.
Carve duck breasts on diagonal and spoon sauce over them. Serve decorated with orange slices and watercress, if desired.
2 tablespoon sugar
2 tablespoon balsamic vinegar
2 tablespoon soy sauce
1 cup orange juice
1 cup chicken stock
2 tablespoon Grand Marnier, or other orange flavoured liqueur
2 tablespoon butter