Salt the fish fillets and marinade them in the juice of one lime / half a lemon with half the grated ginger and 1 finely chopped garlic clove for 30 minutes.
In a large frying pan, heat the oil. Fry the onions until translucent, then add the remaining garlic, chilli, curry leaves and the remaining grated ginger, and fry for one minute. Add the tumeric, stir in the coconut milk and 125ml water and gently simmer for 2 minutes, taking care not to boil. Add salt to taste, then lay the fish fillets and prawns across the bottom of the pan with any remaining marinade. Simmer gently until the fish is tender and cooked through. Remove from heat and stir in the remaining lime/lemon juice. Serve immediately with boiled rice.