1.Rinse rice under cold water until water runs clear; drain.
2.Bring the water to the boil, covered, in medium saucepan; add rice. Cook, covered, over low heat, 10 minutes. Remove from heat; stand, covered, 10 minutes. Fluff rice with fork.
3.Meanwhile, place fish, in single layer, in large baking-paper-lined bamboo steamer; top with lime slices. Place steamer over large saucepan of simmering water; steam fish about 8 minutes or until cooked through.
4.To make tamarind lime dressing; combine ingredients in small saucepan; bring to the boil. Boil, uncovered, about 2 minutes or until thickened slightly.
5.Divide rice among serving plates; top with fish, onion and dressing.