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