Clean and wash the fish thoroughly in water. Apply turmeric and rock salt and wash again in plan water. Lets marinate the fish with turmeric, salt and chilly powder. check measurements given above. Marinate for 1/2 hour.
Meanwhile peel and chop shallots, grate onion and chop tomatoes.
In a clay pot or tawa add oil to fry the fish. fry for 2 mins one side and flip over the other side and fry for another 2 mins and transfer to a plate.
Use the same oil used for fish fry while making the kuzhambu. Add 1/2 tsp fenugreek seeds and curry leaves saute well.
Add chopped shallots and onion saute well. Add 1/2 tsp ginger and garlic paste. Add 2 tsp coriander powder, 2 tsp chilly powder. Add 1 tsp kashmiri chilly powder, 1/2 tsp cumin powder.
Mix everything well. Add 4 chopped tomatoes and saute till mushy. Once tomatoes are mushy add tamarind water and allow to boil till the oil oozes out.
When oil gets seperated around the side Now add the fried fish and switch off the stove. Already the fish is fried it absorbs the kuzhambu well. Rest it for 1/2 an hour and then serve.