Indian recipe: Machchi Do Piaza (Bengali curried fish)

500 g (1 lb) firm white fish fillets
2 tsp ground turmeric
Lemon juice
1 1/2 tsp chilli powder
1 tbs chopped fresh ginger
2 tsp chopped garlic
2 ripe tomatoes, chopped
2 large onions
1/4 cup mustard oil or other vegetable oil
4 fresh green chillies, seeded and sliced
1 tsp salt
1/2 cup water

Wash the fish and rub with half the turmeric and lemon juice and set aside. Put remaining turmeric, chilli, ginger, garlic, tomatoes and one roughly chopped onion into a blender and purée. Slice the remaining onion very finely.
Heat the oil in a saucepan and fry the sliced onion, stirring frequently, until it is golden. Remove from pan and reserve. Put the spice-onion purée into the pan along with the chillies. Stir over medium heat until the colour of the paste darkens and the oil separares from the paste. Add the fish, salt and water and simmer, covered, for about 15 minutes. Do not stir, so as to not break up the fish, but shake the pan occasionally. Add the fried onions after 10 minutes. Serve with rice.