Cut the fish into 2-inch pieces and clean the fish thoroughly.
Add the chilli powder in tamarind juice; add salt, 1 tsp turmeric, green chilies, mix well. Keep aside.
Heat 3 tsp oil in a non-stick skillet.
Add mustard seeds and cumin seeds in the oil.
When it starts to sputter, add the fenugreek seeds; once you get a sweet and sour flavour and the seeds turn golden brown, add curry leaves and onion.
Add a pinch of salt so that the onion gets cooked quickly.
When onion turns golden brown, add tomato and cook for another 2-3 minutes.
Add cumin powder, coriander powder and turmeric powder, cook for a minute.
Add the mixture of tamarind juice,leave it for a couple of minutes till the sauce is well cooked.(You should not get any smell of tamarind once properly cooked).Add a pinch of sugar while the sauce is simmering.
Add fish at the end, cook for another 10 minutes in medium flame.