Sprinkle salt and turmeric powder on the fish and keep aside for 10 mins.
Heat enough oil for deep frying and fry the fish pieces a few at a time till crisp and golden brown.
Remove most of the oil leaving only a 2-3 tsp. in the pan. Remove the oil from fire, mix in the beaten curd. mustard powder and a few slit green chillies (optional). Blend well so that the mustard and curd mix well in the oil.
Add a cup of water. Put the mixture back on fire and add the fish pieces. Boil over low heat for 2-3 mins.