Pour the marinade over the duck and refrigerate for at least an hour.
With oranges, remove peel and white membrane and separate into segments.
Preheat oven to 240C (220 – 240, depending on your oven). Put the duck on a baking sheet skin down, pour half of the marinade and bake for 5 minutes. Turn the duck over, pour over the remaining marinade and bake skin-up for another 15 to 20 minutes (depending on how you like the duck. I prefer it well cooked, without blood, so I cook for 20 minutes). Take out of the oven, cover with foil and let "rest" for 5 minutes. Cut the duck thin enough plates.
For dressing mix soy sauce, vinegar, sugar and sesame oil. Arrange the lettuce leaves on plates, put the duck and orange slices on top. Pour the dressing.