Put the chicken stock and water into a large pan, add the rinsed prawn shells, which will give the soup extra flavour, lemon grass, lime leaves and ginger and simmer for 20 minutes. Strain the stock. Return the stock to a pan and add the fish sauce, lime juice and chilli paste.
Bring to the boil and throw in the garlic and shallots, the chillies, mushrooms and coconut slivers. Cook gently for around 3 minutes.
Add the prawns and simmer slowly, uncovered, for 2-3 minutes, until the prawns turn pink. Serve garnished with coriander.
© 1994 Keith Floyd estate. All rights reserved.