For the curry paste, blend all the ingredients in a food processor until smooth.
Season the cod well with salt and pepper. Heat the oil in a deep-frying pan, add the onion and fry for about 10 minutes, until richly golden. Add the curry paste and fry for 2 minutes. Stir in the tomatoes and fry for 1 minute, then add the coconut milk and bring to the boil.
Add the fish and whole chillies to the pan, cover and simmer for 5-6 minutes, turning the fish over half-way through. Lift the fish out of the pan, put them in a serving dish and keep warm.
Serve in a bowl with a sprinkling of coriander, a whole chilli on top and boiled rice.
