Soak the kokum in one-fourth cup of water for half an hour. Strain the water and keep aside. Grind together garlic, ginger, green chillies, coriander leaves, red chilli powder, turmeric powder and salt with a little water to a thick paste.
Apply this masala to the fish pieces and keep aside for fifteen minutes. Heat the oil in a non stick pan. Add the onions and sauté till golden brown. Add the marinated fish pieces and sauté for a minute stirring lightly.
Add the kokum water and cover and cook on low heat for three to four minutes or till oil surfaces on top and most of the moisture is absorbed. Garnish with coconut, stir lightly and serve hot with rotis.