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1
Preheat the oven to 325.
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2
In a small bowl, combine the granulated sugar with the palm sugar, fish sauce, five-spice powder and ginger.
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3
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.
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4
Using a sharp knife, score the fat cap of the lamb shoulder crosswise at 1-inch intervals, making the cuts 1/2 inch deep.
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5
Lay the lamb flat on a work surface and rub the sugar mixture all over the meat.
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6
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.
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7
Set the trussed lamb in a small roasting pan, fat side up, and pour the vinegar mixture over it.
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8
Roast the lamb for 1 hour, basting after 30 minutes.
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9
Turn the lamb shoulder over and roast for 1 hour longer, basting again after 30 minutes.
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10
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.
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11
Transfer to a carving board and let rest for 15 minutes.
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12
Skim the excess fat from the juices.
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13
Discard the kitchen strings and thinly slice the meat against the grain.
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14
Serve with the juices.
