Marinate fish pieces with half turmeric powder and salt for 10 minutes.
Grind mustard seeds and green chilies with water into a fine paste.
Heat mustard oil in a pan and lightly fry fish pieces on both sides; set aside.
In 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.