1.Preheat oven to 180°C (160°C fan-forced).
2.Dry-fry star anise and coriander in small heated frying pan, stirring, until fragrant. Combine spices with tamarind, sugar, ginger, lemon grass, chilli, garlic, kecap manis and the water in medium jug.
3.Place lamb, in single layer, in large shallow baking dish; drizzle with tamarind mixture. Cook, covered, in the oven, about 2 hours or until meat is almost falling off the bone, turning occasionally.
4.Remove lamb from dish; cover to keep warm. Skim away excess fat from pan juices then strain into small saucepan. Bring to the boil; boil, uncovered, 5 minutes.
5.Steam choy sum until just tender then divide among serving plates. Top with lamb; drizzle with sauce.