For Shorshe ilish (or hilsa with mustard) for four people, you need 500 g (1 lb) of peti pieces. A big fish will give you eight or nine pieces. These should be washed and cleaned thoroughly. Grind 1½ tablespoons of pungent black mustard seeds with a touch of salt and a green chilli. This will dispel the bitterness of the mustard. Indian shops in the West always sell this mustard and it is worthwhile going to the trouble of buying it, for white or brown mustard is no acceptable substitute. The whole point of the dish is its pungency which complements the rich oiliness of the fish. Mustard seeds can be ground in a blender, though not as finely as on a grinding stone. Take 7-8 green chillies (or 2-3 if you cannot tolerate too much) and slit them down the middle. Rub the pieces of fish with salt and turmeric. Heat 4 tablespoons of mustard oil in a karai and add the mustard paste together with ½ teaspoon of turmeric powder. Stir for a couple of minutes and add 250 ml (8 fl oz) of water. As soon as it comes to the boil, gently put in the hilsa pieces and the green chillies. Cover and cook for ten minutes over a medium flame. Uncover and taste to determine how much extra salt is needed. The water should evaporate sufficiently to leave the fish coated in a thick, grainy, yellow sauce when you remove it. If you want a thinner sauce, you can add some more water. Remove from the stove and add a little fresh mustard oil to the fish. Leave covered for a few minutes. When you serve it with plain boiled rice the sharp taste of the mustard will hit the palate. The important thing to remember, I find, is not to overcook the fish. The stomach pieces, in particular, are very soft and oily and too much heat will destroy the texture, if not the flavour. As with most Bengali dishes, there is always room for experiment and adjustments. Some people prefer more of the mustard paste than others, while some love to drown their fish in mustard oil.