Cut the duck in half lengthways and dry with kitchen paper. Combine all the braising ingredients in a large pan and bring the mixture to a boil. Add the duck halves and turn the heat down to a simmer. Cover the pan and slowly braise the duck for 1 hour, or until it is tender.
Skim off the large amount of surface fat which will be left when the duck is cooked, and allow the duck to cool. Once cooled, take the duck out of the pan, remove any lingering surface fat from the liquid and set the liquid aside.
Carefully remove all the bones from the duck, keeping the meat and skin intact. Place the duck halves between 2 pieces of clingfilm and press the meat and skin together. Baste the duck halves with the beaten egg and dust with cornflour or potato flour.
Next set up a steamer, or put a rack into a wok or deep pan, and fill it with 5cm (2in) of water. Bring the water to the boil over a high heat. Put the duck on a heatproof plate and carefully lower it into the steamer or on to the rack. Turn the heat to low and cover the wok or pan tightly. Steam gently for 20 minutes. Allow the duck to cool thoroughly.
The dish can be completed to this stage a day in advance. When you are ready to serve the duck, heat a wok or a large frying pan over a high heat until it is hot. Add the oil, and when it is very hot and slightly smoking, add the duck halves and deep-fry until they are crispy. Remove and drain well on kitchen paper.
Heat some of the reserved braising liquid and serve with the duck as a sauce.