Put fish in cold water.
Add 1/2-cup salt, vinegar and keep for 15 minutes. Remove and cut.
Heat oil in a pan.
Add mustard seeds followed by curry leaves, onion, ginger and garlic.
Fry till onions are golden brown.
Add tomato and fry.
Add coriander powder, chilly powder and turmeric powder and fry well.
Add water with tamarind juice.
Add salt and allow it to boil.
Add fish and cook till done.
Heat coconut oil and add fenugreek seeds. Pour this over the gravy.
Allow to simmer for 1/2 hour on low fire till the gravy thickens.