Wash the spring onions. Cut the dark green part diagonally into rings.
Finely chop the remainder.
Peel and finely chop the ginger.
Wash the tomatoes, cut out the stalk, then plunge into boiling water for 40 seconds. Plunge into cold water then peel off the skins, quarter, deseed and dice the flesh.
Wash the prawns and drain in a sieve.
Wash, halve, deseed and chop the chillies. Saute in a pan of hot oil with the finely chopped spring onions and the ginger.
Pour in the coconut milk and stock and bring to the boil. Reduce the heat and simmer for 10 minutes.
Add the tomatoes and prawns to the soup and heat through for 5 minutes, without boiling.
Season with lime juice and soy sauce.
Scatter the spring onion greens into bowls and pour over the soup. Garnish with chillies to taste.