Marinate fish pieces with half of the turmeric powder and salt for 10 minutes.
Grind mustard seeds and green chilies with a little water into a fine paste.
Heat mustard oil in a pan and lightly fry fish pieces on both sides, then set aside.
In the same oil, temper nigella seeds, add mustard paste, remaining turmeric, salt, and water.
Add fried fish and simmer for 8 minutes until gravy thickens slightly.