Soak the tamarind in 1/2 a glass of hot water for at least 20 minutes. Make a puree of it. Meanwhile, cut the fish into fist size pieces and clean the pieces with some salt, butter milk or curd and turmeric powder (1/2 tsp) and then with water.
Mix the fish pieces in a bowl with chilli powder, 4 pinches of turmeric powder, salt and ginger-garlic paste.
Heat oil in a non-stick cooking dish and add the ground onion. Fry it on a medium heat till the onion becomes light brown.
Now pour the tamarind puree and add the chillies. Empty the bowl of fish pieces into the dish. Add salt according to your taste.
Let the gravy cook on a low heat. In between, keep moving the dish holding the sides so as to stir the contents. If a spoon is used to stir the gravy, the fish pieces will split and dissolve in the gravy. Instead, a wooden spatula can be used by taking care that the pieces are not disturbed much.
Add garam masala and finely chopped coriander 2 minutes before removing the dish from the flame.
Those who do not like the gravy to be much hot and sour, can reduce the quantity of chilli powder, garam masala and tamarind. Tastes best with rice.