Marinate fish with ground turmeric, chilli powder, and enough salt for one hour. Then heat oil in a pan and shallow fry the fish pieces in batches. Keep aside.
Add the remaining oil into the pan, and splutter mustard, fenugreek, and cumin seeds. Add shallots, green chillies, and curry leaves. Fry until the shallots become golden brown in color.
Throw in ginger garlic paste, and fry until a nice aroma is released. Now you can add asafetida, ground turmeric, ground coriander, and chilli powder. Fry for a few seconds, and season with black pepper. Throw in tomatoes and cook. When oil separates, add tamarind extract. At this stage, add fish, cover it with lid and cook. When the fish is cooked, check the seasonings and add vinegar. Allow the fish curry to boil. Add coconut milk and remove from flame. Serve hot.