For the boiling marinade: Put all of the ingredients in a tall pot, such as an asparagus steamer, and bring to the boil. Boil for ten minutes and then reduce to a simmer.
Meanwhile, cut the wings tips off the duck and make two cuts into the carcass, parallel to the wing bones. This will allow the duck fat to escape during roasting.
Suspend the duck, using a butcher's hook or similar into the neck over the pot, without letting it fall into the marinade. Using a small soup ladle, pour the marinade all over the duck. Keep doing this until the duck has a nice golden yellow color from the tumeric. Place the duck on a rack in a roasting pan and dry for approximately one hour.
After the duck has dried, preheat the oven to 200C (400F) and roast the duck on a rack over a roasting tin of water for approximately one hour and a half. Check the duck half way through cooking because you may need to put a tent of aluminum foil over it to prevent the duck breast and wings from overcooking.
While the duck is roasting, prepare the sauce. Put all of ingredients in a small saucepan, except for the cornstarch and water. Bring to a boil and reduce the sauce by half. Then, make a slurry of cornstarch and water, and whisk it into the sauce to thicken it. When the sauce is sufficiently thickened, take it off the heat and reheat it before serving.