Lightly roast cumin seeds and coriander seeds. Grind them into a powder with dried red chillies and turmeric.
Cut boneless fish into one inch sized cubes. Apply salt. Apply ground chilli, turmeric, coriander and cumin masala.
Peel onions and chop one onion and grind the other onion with grated coconut, peppercorn into a coarse paste. Finely chop tomato and keep aside.
Heat oil in a pan, add chopped onions. Sauté till onions turn pink. Add chopped tomatoes and cook till oil leaves the sides.
Mix in ground coconut masala paste and cook on a high flame for a minute.
Add two cups of water and bring it to a boil. Add fish pieces and cook on medium flame for five to six minutes. Stir in tamarind dissolved in quarter cup of water. Simmer for five minutes.
Serve hot with steamed rice.