Lightly roast cumin seeds and coriander seeds and grind them into a powder with dry red chillies. Apply salt, turmeric powder and the ground powder to the fish cubes and keep aside. Chop one onion.
Soak tamarind in two tablespoons of water and squeeze to get pulp. Grind the other onion with coconut and black peppercorns into a coarse paste.
Heat oil in a pan, add chopped onion and sauté till lightly browned. Add tomato and sauté till oil leaves the sides.
Add the coconut paste and cook on high heat for a minute. Add two cups of water and bring the gravy to a boil.
Add fish pieces and cook on medium heat for five to six minutes. Add tamarind pulp, adjust salt, stir and simmer for five minutes.
Serve hot with steamed rice.