1.Dry-fry seeds, cardamom, chilli, turmeric, cinnamon, cloves and star anise in heated small frying pan, stirring, about 2 minutes or until fragrant. Blend or process spices with onion, garlic, ginger, nuts, yoghurt and juice until mixture forms a paste.
2.Using sharp knife, pierce lamb all over; place on oiled wire rack in large shallow baking dish. Spread paste over lamb, pressing firmly into cuts. Cover; refrigerate overnight.
3.Preheat oven to 200°C (180°C fan-forced).
4.Remove lamb from refrigerator; pour enough water into baking dish to completely cover base. Cover dish with foil; roast lamb 30 minutes. Reduce oven temperature to 150°C (130°C fan-forced); roast lamb, covered, further 1½ hours. Uncover; roast about 30 minutes or until lamb is cooked as desired. Cover lamb; stand 10 minutes.
5.To make coriander yoghurt; combine ingredients in medium bowl.
6.Serve sliced lamb with yoghurt and, if desired, pilaf.