Marinate fish with half turmeric powder and salt for 10 minutes.
Grind mustard seeds with green chili and 2 tbsp water into a smooth paste.
Heat mustard oil in a pan, light-fry fish for 1 minute per side; remove.
Temper nigella seeds in oil, add mustard paste, remaining turmeric, salt, and water.
Add fish pieces and simmer gently for 6 minutes before serving warm with rice.