Soak tamarind in hot water for 15 minutes. And extract the juice by squeezing with the hand.
Transfer the extracted tamarind to a bowl and set aside.
Fro grinding
Over medium heat, in a pan, add vegetable oil, toor dal, chana dal, red chilies, coriander seeds, black pepper, cumin seeds, fennel seeds, and coconut slices.
Dry roast all the ingredients evenly. Allow it to cool down.
In the blender jar, add all the dry roasted ingredients and water.
Grind them to fine paste and set aside.
For seasoning
Over medium heat, in a pan add sesame oil, black mustard seeds, urad dal, and fenugreek seeds.
And allow the mustard seeds to splutter.
Then add onion, saute till it becomes soft and translucent.
Now add kept aside ground masala and continue stirring them until the masala thickens.
For the curry
Then add tomatoes and continue cooking until tomatoes become soft and pulpy.
Now add tamarind extract, water, and salt.
And bring it to boil for about 10 mins until it leaves out the raw flavor.
Now you may adjust water and salt according to the desired level.
Then add fish pieces, in low heat cook for another 10 minutes.