Heat oil in meen chatti add mustard and fenu greek . when it splutters add onions, stir for 3,4 minutes.
Add ginger, garlic and green chillies sauté for 2 minutes.
Add tomatoes, chilli powder, turmeric powder and coriander powder sauté for a minute.
Add 1 cup water and boil for 2 minutes.
Then add fish pieces to this gravy and mix well.
Close meen chatti with a lid and cook for 10 minutes until fish is cooked and gravy becomes thick.
Then add salt and kudam puli to this and cook for another 5 minutes in low flame.
Now Kerala fish curry is ready to serve with rice or kappa.