Cover the dried mushrooms with boiling water, let them soak for an hour until soft, remove and discard stem and cut them into quarters.
Cut the whites off the spring onions and gently crush with a Chinese cleaver or knife. Cut the green parts into rings and set aside for garnish.
Heat the oil in a wok over a high flame. Add the ginger and spring onion and stir-fry until fragrant. Remove from the wok into a bowl, add the duck to the wok and fry until brown on all sides.
Add all the other ingredients, including the reserved spring onion and ginger – except the reserved spring onion greens and sesame oil. Bring liquid to the boil, then cover and simmer gently for an hour.
Remove the duck, mushrooms and chestnuts from the wok and place in a serving bowl. Simmer the sauce until it slightly thickens, then add sesame oil and pour over duck. Sprinkle with spring onions. Serve with boiled or fried rice and steamed greens with oyster sauce.