Place the prawn heads and shells into a saucepan, add the oil and onion and fry for 2-3 minutes.
Add the lemongrass, half of the tomatoes and all of the boiling water. Simmer for ten minutes.
Strain through a sieve into a clean pan. Add the soy sauce, mushrooms, remaining cherry tomatoes and prawns and simmer for three minutes, or until the prawns have just turned pink.
Stir in the chives and finger lime seeds. Pour into a serving bowl and garnish with chopped coriander.
