Apply a pinch of salt and turmeric powder to the fillets, mix evenly and keep it aside.
In a frying pan, pour 4 tablespoons of oil, and add the fennel seeds and fenugreek. Sauté till aroma (about 1 min). Add onions and garlic. Fry till the onion becomes slight golden brown. Add the tomatoes, red chile powder and coriander powder and saute it for 2 minutes, till the tomatoes are pulpy.
At this point, add 3 cups of water. (The amount of water depends on the density of gravy needed). Now add the tamarind extract, and let this mixture boil on low flame for about 10 minutes.
Add the coconut milk and continue in low flame for another 5 minutes. Now the mixture would have reduced considerably and will be quite thick. Add some more water, if you want.
Add the fish pieces and some salt (keep in mind that you have already added salt to the fish) as required . The fish will cook very quickly, in a matter of five minutes. Don't overcook the fish, as they may break when served.
Serve it hot with rice. Note: The more thicker, the more better. It gets thicker as time goes by.