To make Ilish macher Jhal, start by making a thick paste of Mustard seeds with 2 green chilies and ½ Tsp. of salt.
Strain the mixture to discard the husk and set aside.
Rub fish pieces with salt and turmeric powder and keep aside.
Heat Mustard oil in a deep pan and add turmeric powder, red chili powder, and salt.
Temper hot oil with Nigella Seed. Pour in the mustard paste, green chilies, red chili powder, and water and boil on medium flame.
Add the fish pieces and cook on low flame for at least 10 minutes.
Cover the pan with a lid.
Uncover the pan and pour raw Mustard oil over the gravy and boil for 2-3 minutes. Adjust the consistency and serve with steamed rice.