Marinate the fish pieces with chilly powder, turmeric powder, ginger & garlic paste and salt. Keep it aside for 10-15 minutes.
While fish is getting marinated, take a meen chatti(flat earthen pot) or if not available use a normal non-stick vessel and heat up 1 tsp of oil(coconut oil preferred but any oil will do). Use slow to medium flame throughout the cooking. Add chopped tomatoes & chillies. Fry it in slow flame for 10 minutes(make sure it is not burned and keep stirring). Then add the tamarind paste or green mango. Add marinated fish in the mix. Add hot water(enough to cover the fish) and cover the meen chatti. Let it cook for another 15-20 minutes. Only use medium flame.
While fish is being cooked, take another cheena chatti (or just a frying pan) and heat up 2 tsp of oil(any oil will do but coconut oil preferred) and add small onions, mustard seeds(kaduku), fenugreek(uluva) seeds and curry leaves. Use medium flame and fry till onion turns golden brown.
By now fish would have cooked (after 20 minutes of cooking in meen chatti) and the gravy would have thickened nicely. Make sure fish pieces are not broken. Reduce the flame to minimum and add thick coconut milk. Stir it nicely. A golden yellow color will come. Let it boil for 1, 2 minutes in slow flame.
Add the fried onion, mustard seeds, fenugreek seeds and curry leaves. Mix and stir well.
The curry will look golden yellow and will have a coco nutty, hot, sour taste. You can alter the chilly, tamarind and salt as per your personal preference. Serve this hot curry with boiled white rice.
Tip:
While eating the rice and fish curry, pour a liberal amount of moru(butter milk) or yoghurt (thairu) and mix well. Trust me it tastes like heaven!