Cut the duck in half lengthways, using a meat cleaver and poultry scissors. Place the halves in a wok and pour in the measurement water, then add 1 tablespoon of the soy sauce. Put the lid on the wok and bring to the boil. Reduce the heat so that the liquid simmers steadily and cook for 1 hour.
Prepare the pineapple while the duck is cooking: trim the leaves off the top and cut off the stalk end. Cut off the peel and cut out all the spines, then slice the fruit in half lengthways and remove the hard core. Cut the pineapple halves into slices and set them aside.
Remove the duck from its stock using a slotted spoon or tongs and set aside. Pour the stock out of the wok and wipe out the wok. When the duck is cool enough to handle, cut all the meat off the bones and slice it into pieces.
Heat the sesame oil in the wok over a high heat until the oil starts to shimmer. Add the chillies, garlic and duck meat and stir-fry until lightly browned, then add the water chestnuts and pineapple and cook for 1–2 minutes. Stir in the remaining soy sauce and any juice from the fruit, and sprinkle with the spring onions. Cook for 1 minute and serve immediately.