Marinate fish with half turmeric powder and salt for 10 minutes.
Heat mustard oil in a pan and light-fry fish steaks for 1 minute per side; set aside.
Temper cumin seeds in the same oil, add ginger paste and chopped tomatoes; cook until soft.
Add remaining turmeric powder, salt, slit green chilies, and water; bring to a boil.
Add fried fish pieces, cover, and simmer for 8 minutes before serving.