Remove head, fins and tail from fish and cut into 8-10 large pieces. Combine fish, nuoc cham, pepper and spring onion, allow to marinate for 15 minutes.
Place water in a large saucepan and bring to the boil. Add the fish with its marinade and lemongrass. Reduce heat and simmer for 20 minutes.
Meanwhile, combine tamarind pulp and boiling water and allow to soak for 15 minutes. Strain mixture through a fine sieve and discard pulp.
Add the tamarind liquid, sugar, bamboo shoots, pineapple and tomatoes to the pan. Simmer for 4-5 minutes until fish is tender.
