Set an earthen cooking vessel or kadai or a pot over medium high heat and add coconut oil.
When the oil becomes moderately hot throw in the fenugreek seeds, mustard and the curry leaves and allow them to sizzle just for a few seconds.
Now, add the pounded onion, ginger and garlic mixture and fry very well stirring continously until oil begins to leave the sides of the cooking vessel. At this time you may add salt. Also, you will begin to get a lovely aroma at this stage.
Add the dry spices and roast them well until you see streaks of oil appearing. You may sprinkle a few drops of water to help in the roasting process.
Pour about 1 cup water. Give everything a very good mix. Cover and bring the curry to one boil.
Open the lid and mix in the tamarind water first and than the coconut milk. Mix well.
Add the fish pieces. Coat them well with the curry sauce. Cover and cook over medium low heat until the fish pieces are just cooked through. Took be approximately 5 mins.
Switch off the heat. Allow the Kearala Fish Curry to rest for a while. I love to wait for atleast 2 hours for the flavors to mature. It certainly tastes the best the next day. Enjoy with hot white rice.