1.Remove meat and skin from duck. Remove and discard excess fat from meat and skin; reserve half the skin. Slice duck meat thinly.
2.Roughly chop duck bones and place in large saucepan with stock, the water, star anise, cinnamon, garlic and ginger. Bring to a boil; boil, uncovered, for 20 minutes. Strain liquid into another saucepan; return liquid to a boil.
3.Place noodles in medium heatproof bowl, cover with boiling water, stand for 5 minutes or until just tender; drain.
4.Add duck, sesame oil, half of the gai larn, half of the onion and half of the coriander to the broth; stir until hot.
5.To serve, divide noodles among bowls; top with broth, remaining gai larn, onion and coriander, then top with finely sliced reserved duck skin.
The stock mixture can be made a day ahead; complete soup just before serving. Chinese barbecued duck can be purchased from Asian-food stores.Note