Microwave tamarind with water for 5 min or soak it in warm water. Extract the pulp and add turmeric powder, coriander powder and chilli powder. Mix well to a paste
Heat oil, crackle mustard seeds, fenugreek seeds and cumin seeds.
Once it splutters, add in finely chopped shallots, finely chopped ginger and garlic and green chilli slit. saute till onion is pink.
Add in curry leaves and chopped tomato. cook till tomatoes are mushy.
Pour in the tamarind masala mix paste and little water. Mix well. let it boil until oil leaves the sides.
Cool it down and take it in blender. Grind it till creamy and smooth.
heat oil again, crackle mustard seeds, Pour in the ground masala and salt. Close and cook again till it thickens. Add thick coconut milk and fish pieces to the curry.
Close and let it stand for 5-10 minutes or until fish is cooked. Put off the stove and let it sit for an hour minimum before it is served. The essence of masalas and tamarind will get into fish by then.