1.Trim and discard excess fat from lamb. Cut lamb into two even-sized pieces; place in a large shallow dish with combined sauce, sugar, oil, ginger and garlic. Cover; refrigerate 3 hours or overnight.
2.Drain lamb over a small bowl; reserve marinade. Cook lamb on heated oiled barbecue until browned all over. Place lamb in a disposable baking dish; cook in a covered barbecue, using indirect heat, following manufacturer’s instructions, about 30 minutes or until cooked as desired. Remove from barbecue. Cover; rest 10 minutes.
3.Meanwhile, make coriander potatoes. Boil, steam or microwave potato until tender; drain. Place potato in a large bowl with combined oil and coriander; toss gently to combine.
4.Bring reserved marinade and the water to the boil in a small saucepan. Reduce heat; simmer, uncovered, 5 minutes. Strain into a small jug.
5.Serve lamb with potato; accompany with sauce.
The lamb can be cooked in the oven, if you prefer. Preheat oven to 180°C/350°F and roast, uncovered, for about 30 minutes.Note