Heat up a deep botton saucepan/ wok/ kadai. Add mustard oil and let it heat up properly. Fry the marinated fish both the sides well to crisp and brown. Take out and let it cool a little bit.
Now if you have some oil still left after frying, then do not throw it, add 2 tsp mustard oil more.
Temper this oil with fenugreek seeds and green chillies.
Then add all the masalas with some water. Fry it for 2 minutes at mid heat.
Now add 2 cups plain water and let the masalas cook for 5 minutes. stir occasionally.
Add the fried fish to the gravy. Cover it and cook for further 6-8 minutes or till the fish becomes soft.