Soak tamarind in water for 10-15 minutes. Strain the pulp and discard the hard bits. Set aside.
To grind coconut:
Place all the ingredients to grind except water in the bowl of a food processor. Add a little water, and process until smooth. Set aside.
To make the curry:
Heat oil in a large pan over medium heat. Add onion and a sprig of curry leaves. Saute for 2-3 minutes or until onions are softened. Add green chili and tomatoes. Saute for 2-3 minutes, or until tomatoes start to break down.
Stir in coconut paste and enough salt. Fry for 1-2 minutes. Add the tamarind pulp and enough water to make a thick sauce. Bring to the boil. Slide in the fish pieces and add the rest of the curry leaves. Bring to the boil again. Simmer for 7-10 minutes or until fish is well cooked. Garnish with coriander leaves.
Serve Goan fish curry warm with rice or any flatbread.