Fry the prawns in oil for about 1 minute on each side. Bring coconut milk and fish stock to the boil and simmer for 2-3 minutes. Peel ginger and grate it into the soup. Clean the chilli, remove the seeds, cut the pod into fine rings and add to the soup.
Cut the lime in half. Cut one half into narrow slices and squeeze the other half. Season soup with fish sauce, lime juice and salt. Clean and wash spring onions and cut into wide strips.
Add spring onions and shrimps to the soup and simmer for 3-5 minutes. Wash coriander, pluck leaves from the stalks, garnish soup with coriander leaves and lime slices.