In a bowl take katla fish, add turmeric powder and salt as per taste and keep it aside.
In a pan dry roast cumin seeds and coriander seeds.

Heat oil in a pan, add chopped onions, green chillies and fry them. After they get fried, add them in a grinder.

Also add the dry roasted cumin and coriander seeds in the grinder. Grind them.
In a pan heat oil, add mustard seeds, fennel seeds and the grinded paste.

Into it add curry leaves, turmeric powder and ginger garlic paste.

Also add red chilli powder, tamarind paste(soak tamarind in a bowl and make a paste).

Add salt as per taste and fry the masala well. Until oil gets separated.

Add water(add water as per required, only for cooking the fish), let it boil, then add the fish and chopped coriander leaves. Cook it well.