Marinate the fish with a pinch of salt and turmeric powder for 5 minutes. Heat oil in a pan and fry them on both sides till golden in colour. Drain and keep aside.
In a bowl, mix together mustard paste, turmeric powder and tomato paste with little water to form a paste. Keep aside.
Temper the same oil with nigella seeds. Saute for a few seconds and add the mustard paste. Saute till the oil separated and then add 1 cup water.
Bring it to a boil. Then slowly drop in the fried fish, salt and the green chilies. Simmer on a medium flame for 2-3 minutes.
When done, add the raw mustard oil and coriander leaves and switch off the flame. Serve with plain steamed rice.
