1. Cut fish fillets into 5 cm (2 inch) squares and place in a dish. Thinly slice one onion and Set aside. Mince ginger, lemon grass, garlic, dried shrimps and desiccated coconut to a fairly smooth paste and add thick coconut milk. Pour over the fish and marinate for 1 hour.
2. Heat oil and fry sliced onion until lightly colored. Add fish pieces and cook briefly, then pour on thin coconut with coriander and marinade. Cover and simmer on low heat until fish is cooked. Lift out fish with a slotted spoon and place on a serving dish.
3. Continue to simmer sauce until thick and creamy. Season to taste with lemon juice or tamarind water and add salt. Pour over the fish and serve at once.