Clean the fish and cut into 4-inch pieces. In a blender, puree the coconut, cumin seeds, coriander seeds, red chilies, and turmeric using the coconut milk. Heat oil in a pan and fry the onions until brown. Add the tomatoes, curry leaves and fry for 2 minutes. Now add the coconut puree and the tamarind paste and mix well. Cook for 5 minutes by stirring constantly. Then add the fish pieces and salt and mix thoroughly. Simmer for 15 minutes, uncovered until the fish is done. Serve hot with rice.