Wash the fish pieces only once under running water. If you wash hilsa too much, the flavour reduces.
Marinate the pieces of fish with salt and 1 tsp of turmeric powder and keep aside.
In case you have a mustard paste of black mustard, make sure to strain it properly to remove the black skins or else the paste kind of becomes bitter.
In a wok or a frying pan, pour a tbsp of mustard oil and lightly fry thefish pieces. With Hilsa, one doesn’t need too much oil since the fish by itselfis very oily and some of them ooze out oil.
This should not take more than a minute and half. It is best to just place the fish in the pan with the hot oil and flip within 30 – 40 seconds keeping the other side for an equal amount of time.
To prepare the sauce/ gravy, using the oil from the pan, plus another couple of table spoons of mustard oil. When the oil is hot, add the nigella seeds and a couple of whole green chilies (break them into two) and let them splutter.
Immediately add the mustard paste, turmeric powder and red chili powder and season with salt.
Keeping the heat at a medium, cook the mustard paste till the raw smell from the turmeric goes off. Sprinkle some water if you like to keep the paste from sticking to the pan.
Beat the hung curd in a bowl and then add to the sauce. Remember to lower the heat or else the curd will split. Add a little bit of water and cook on a low heat for about 5 minutes.
Add the fish pieces along with the pan juices from the fish. Add the desired amount of water and let the gravy simmer.