Bring the stock to the boil in a large saucepan and add the lemon grass, lime leaves, mushrooms, fish sauce and chillies. Boil for 2 minutes.
Add the prawns, spring onions, lime juice, curry paste and sugar to the saucepan and cook for 2 minutes or until the prawns turn pink. Add the coriander just before serving.
Did you know?
Chillies are a leading source of vitamin C. They're not usually eaten in large quantities – but the more, the healthier. * Red chillies contain more than ten times the beta-carotene of green chillies. * Capsaicin acts as an anticoagulant and thus may help prevent heart attacks.