Wash the fish pieces thoroughly till they are white, drain water and keep aside.
Finely chop onions.
Take a thick bottomed wide vessel and add oil and let it heat up.
Once the oil is hot enough, add mustard seeds and jeera and let them splutter. Add the onions and fry till transparent.
Add turmeric, curry leaves, slit green chillies and sauté for a minute.
Add ginger-garlic paste and mix well and cook till the raw flavour is gone.
Add tomatoes and cook till soft.
Add salt and mix thoroughly.
Wash the tender tamarind leaves well and pat them dry with a soft towel. Now using your both hands crush them gently and keep aside.
Add this crushed tamarind leaves, garam masala and cook for 10 mins till oil oozes out.
Add enough water and mix well. Do not add too much water.
Now slowly place the fish pieces in the pan. Spread them evenly around the pan.
Place the lid on and cook for 10 – 15 mins. Check the consistency and if you think it has become thick, add little water and just move the pan around without using a spatula. The fish pieces might be disturbed if you use a spatula. However, if you are experienced in cooking fish, you can use it.
After 15 mins, slowly turn over the fish pieces to the other side and cook for 10 more minutes. Actually, when we add tamarind juice the fish pieces are all dipped in the gravy due to the tamarind juice and water that we add. Here as we are using tender tamarind leaves not much water is present in the curry.
Simmer till the fish absorbs the gravy and is cooked properly.
Garnish with finely chopped coriander.