step 1 Bring the stock to a boil in a medium-sized saucepan. Add the lemongrass, galangal, coriander roots and lime leaves, then simmer for 2 mins. step 2 Add the prawns, fish sauce, chillies and lime juice, then return to the boil. Taste and adjust the seasoning with either more lime juice or fish sauce, then garnish with coriander leaves and serve.