2.5 cm (1 inch) piece of fresh root ginger, peeled and cut into matchsticks
6 garlic cloves, finely chopped
1 kg (2 1/4 lb) skinless halibut fillets, boned and cut into bite-sized pieces
400 ml (14 fl oz) can coconut milk
300 ml (1/2 pint) water
1 tablespoon tamarind paste
steamed basmati rice, to serve
Mix together the turmeric, chilli powder and coconut in a small bowl and set aside.
Heat the oil in a large wok or heavy-based saucepan until hot, then add the mustard seeds and cook over a medium-high heat for a few minutes until the seeds begin to pop, then add the curry leaves, onions, green chillies, ginger and garlic and stir-fry for about 5 minutes until fragrant.
Stir in the turmeric mixture and stir-fry for a further 1 minute. Add the fish, then stir in the coconut milk and measurement water. Finally, add the tamarind paste. Bring to the boil, then reduce the heat to low and simmer gently, uncovered, for 15 minutes or until the fish is cooked through. Season well with salt.
Ladle into warm bowls and serve with steamed basmati rice, if liked.