Soak dry tamarind in ½ cup of warm water for a few minutes.
Extract the tamarind water by squeezing nicely tamarind in warm water.
Strain the tamarind water and keep it aside. Add 100 ml cup of water again to squeeze the remaining extract from the tamarind.
Strain again and keep it aside. Use that tamarind water. Note: If using tamarind paste (thick double concentrated, bought from the shop) just add water as per the curry requirement.
Use the tamarind as per the sourness you like.
Heat 2 tsp of oil in a vessel and add the mustard seeds, methi seeds, dry chillies and curry leaves and let them splutter for few seconds.
Add the chopped onions and fry on medium heat till the onions turn transparent. Add ginger-garlic paste and fry for 1 minute.
Add turmeric powder, cumin powder, red chilli powder and chopped tomatoes. Cook on a medium flame for 4 to 5 mins or until the tomatoes are soft.
Add 250 ml of water, let it boil for 5 to 8 minutes.
Add 100 ml of the tamarind extract and 250 ml of water, bring it to boil and let it boil for 5 minutes.
Now carefully place the fish pieces in the vessel, add green chilli, and coriander leaves.
Shake the vessel well instead of mixing it with a spoon. So that the fish pieces are not broken.
Cook for 10 to 12 minutes by closing the lid, till the fish curry is cooked, in between rotate the vessel, and shake it slightly.
Cook till the consistency of the curry forms and rawness of the tamarind disappears.
Reduce the flame, add salt, cover and cook on low heat for 10 to 12 mins. Let the sauce thicken and till the oil separates.
Adjust salt and garnish with chopped coriander leaves again.
Serve this delicious Andhra chepala pulusu (fish curry) with plain rice or ragi sankati/ragi sangat.