Rub the fish pieces with salt and set aside for 5 minutes. Then rinse under cold water. Grind the coconut with little warm water and grind it into a smooth paste. Strain and reserve the coconut milk. Heat the oil in a pan and fry the ginger, green chilies and onion and fry until the onion is golden. Add the turmeric and coriander powder and stir once. Add the cornstarch to the coconut milk and mix well. Add this mixture to the pan and bring it to a boil. Then add the fish pieces and salt and cook for 15 minutes or until the fish is tender. Add the vinegar and cook for couple more minutes. Sprinkle the curry leaves on top and serve hot.