Mix vinegar, lemon juice and salt together and marinate the fish in this solution for 30 minutes.
Pound the cumin seeds and fenugreek seeds together and set aside.
Soak the tamarind in 1 cup of water for 20 minutes. Strain and set aside.
Grind ginger, garlic, poppy seeds, and tomatoes with coconut without adding water for couple of minutes.
Heat oil in a large pan and add the mustard seeds and when they pop add the onions and fry until light brown. Add the curry leaves and mix. Then add the red chili powder, turmeric powder and coriander powder and stir-fry for a minute. Now add the coconut mixture and stir constantly for 5 minutes. Add the tamarind water, salt and 2 1/2 cups of water and bring it to a boil. Then add the fish gently and put the fenugreek and cumin powder. Cook over low heat until the fish is cooked.