Fry thinly sliced onions, ginger, garlic, curry leaves and green chillies.
To this add peeled and chopped tomatoes, fry well and add red chilli powder, coriander powder and cumin powder and turmeric.
Once the masala is well fried add hot water and make a gravy.
Bring it to a boil.
Slip in the cleaned and sliced fish and cook till the fish is done.