How to Make It
Preheat the oven to 325°. In a small bowl, combine the granulated sugar with the palm sugar, fish sauce, five-spice powder and ginger. In another bowl, combine the water with the vinegar, soy sauce, chile paste and shrimp paste; whisk until no lumps of chile paste or shrimp paste remain.
Using a sharp knife, score the fat cap of the lamb shoulder crosswise at 1-inch intervals, making the cuts 1/2 inch deep. Lay the lamb flat on a work surface and rub the sugar mixture all over the meat. Roll up the roast, wrapping the flaps of fat around the outside to form a neat cylinder, then tie at 1-inch intervals with kitchen string.
Set the trussed lamb in a small roasting pan, fat side up, and pour the vinegar mixture over it. Roast the lamb for 1 hour, basting after 30 minutes. Turn the lamb shoulder over and roast for 1 hour longer, basting again after 30 minutes. Turn the lamb once more and cook for about 45 minutes longer, basting once, until an instant-read thermometer inserted in the center of the roast registers 140° for medium-rare. Transfer to a carving board and let rest for 15 minutes. Skim the excess fat from the juices. Discard the kitchen strings and thinly slice the meat against the grain. Serve with the juices.
Kale-and-Shiitake Fried Rice or steamed white rice.