Bring a large pan of water to the boil. Trim off the visible duck fat, then prick the skin all over, piercing through the fat but not through the meat. Plunge the duck into the boiling water and blanch for 1 minute. Drain and dry on kitchen paper.
Stir together the hoisin sauce, five-spice powder, root ginger, spring onions, sherry and yellow bean sauce, and spread on the underside of the duck. Place it, skin side up, on a rack set over a roasting tin.
Put the honey, soy sauce and water in a small saucepan and bring to the boil. Pour this mixture over the duck, collecting the juices in the tin. Pour the mixture back over the duck twice more, then leave the duck, on the rack, in a cool, draughty place for about 5 hours. Alternatively, leave it in front of a fan, or in a fan oven with just the fan turned on and no heat, for 2 hours. The skin of the duck should dry out and look a bit like baking parchment. Reserve the honey and soy mixture.
Preheat the oven to 200 C / Gas 6. Pour about 300ml of water into the roasting tin.
Roast until the skin is very crisp and brown, about 1 hour.
Pour the reserved honey and soy mixture into a small saucepan and bring to the boil. Simmer for 5 minutes to make a sauce to serve with the roast duck.