To begin, marinate the fish. Add salt, Kashmiri red chilli powder, turmeric powder, and mustard oil. Mix until the fish pieces are well coated. Leave aside to marinate.
In the meantime, prepare the masala. Grind 4 tbsp mustard seeds with a bit of water to make a smooth paste. Remove to a bowl.
Add cumin powder, coriander powder, 1 tsp Kashmiri red chilli powder, and garlic paste. Mix well and keep aside.
Heat mustard oil in a pan to the smoking point and let it cool a bit. Shallow fry the fish pieces until cooked. Remove and keep aside.
In the same oil, add the mustard and fenugreek seeds. Allow them to crackle.
Add in the masala paste you prepared earlier. Saute for a minute on medium to low heat until the liquid dries a bit.
Add the chopped tomatoes with salt, Kashmiri red chilli powder, and turmeric powder. Cook until the tomatoes soften and oil begins to appear on the sides. Add a bit of water if required.
Now, add the fish pieces.
Add about 2 cups of water to form a gravy. Allow it to come to a boil. Lower the heat and let it cook for a few more minutes. Adjust the gravy as par your taste. Check the salt and add more if required.
Finish with chopped green coriander leaves and garam masala powder. Mix well.
The Jharkhand Style Fish Curry is ready to serve. Serve hot with boiled rice.