Remove gills, scales, fins and alimentary canal of all the fishes. Wash thoroughly and cut into medium sized pieces horizontally. Add turmeric powder, red chilly powder and one tea spoon of salt. Mix well and leave for marination.
Add all the spicy ingredients to a blending jar and grind into a smooth spicy paste.
Heat oil in a deep bottomed pan. Splutter mustard seeds. Add onion pieces and fry till the pieces turn brown.
Add the marinated fish pieces and sufficient salt. Mix well carefully and add two cups of water. Turn the fish pieces up and down and allow to cook in low flame until all the ingredients are blended well and the gravy thickens.