Soak tamarind in water for 30 minutes and get a thick juice.
Dry roast the ingredients under the section "-- Dry roast and grind to paste ----" and cool it down to room temperature. Now grind this to thick fine paste by adding water as needed. Add to this ingredients under the section "-- Grind to paste -----" and grind to a fine paste. NOTE its good to grind this masala with "Ammi (grindstone)"
Add the ground masala paste to the tamarind juice and mix well.
In a pan heat oil. Now add mustard seeds, curry leaves. Once the mustard seeds stop splattering add the shallots (sambar onion) and sauté.
Once shallots (sambar onion) is sautéed now add tomato and sauté until the oil separates.
Now add the tamarind juice with masala, salt and water as needed. Cook this until oil starts to separate.
Now add the fish pieces. lower the heat and cook for 10 minutes or until the fish is cooked then serve.