Marinate fish steaks with half turmeric powder and salt for 10 minutes.
Heat mustard oil in a pan until smoking, lightly flash-fry fish for 30 seconds per side; remove.
In the same oil, temper nigella seeds and slit green chilies.
Add remaining turmeric powder, salt, and water; bring broth to a boil.
Add fish pieces, cover, and simmer gently for 8 minutes before serving with rice.