Marinate the fish with a pinch of salt, turmeric powder, chili powder, lime juice and 1 tsp. mustard oil for 10 minutes.
Grind the mustard seeds along with the green chilies, garlic and tomatoes to a paste. Keep aside.
Heat remaining oil in a pan and shallow fry the fish pieces till light brown on both sides. Drain and keep aside.
Temper the same oil with panch phoron, whole dry red chilies and bay leaves. Saute for a few seconds and then add the onion. Fry till light brown.
Add the ground paste, chili powder, cumin powder and turmeric powder. Saute till oil leaves the sides of the pan.
Add 1 & 1/2 cups water and bring it to a boil. Add salt, green chilies and the fried fish pieces. Simmer on low flame till the gravy is slightly thick.
Add coriander leaves and ghee. Switch off the flame and transfer to a serving dish. Serve hot with steamed rice.