To make the marinade, mix all the ingredients together in a bowl.
Cut the skin and fat off the duck breasts and discard. Slice the flesh into thin strips and add them to the marinade. Stir to mix well then cover and leave to marinate for 30 minutes, or refrigerate until required.
Remove the duck strips, reserving the marinade, and pat them dry. Heat the oil in a wok or large nonstick frying pan over a high heat, then stir-fry the duck for 4-5 minutes until browned.
Pour the marinade onto the duck and allow it to bubble until it reduces and becomes dark and syrupy.
Add the spring onions and watercress, stir-fry for 1 minute, then toss in the orange segments and heat them through.
Variation: replace the watercress with shredded Chinese leaf, ribbons of courgettes and carrots, or baby spinach.