1.Combine the water, stock, lemon grass, ginger, chilli and soy sauce in large saucepan; bring to a boil. Reduce heat; simmer, uncovered, about 5 minutes.
2.Meanwhile, make chicken dumplings. Combine ingredients in medium bowl. Using hands, roll level tablespoons of the mixture into balls.
3.Add chicken dumplings to simmering broth; simmer, covered, about 5 minutes or until dumplings are cooked through.
4.Add juice, fish sauce, choy sum and onion to broth; cook, uncovered, until choy sum just wilts. Stir in coriander just before serving.