In a non stick pan, dry roast (on low flame) the shredded/grated coconut along with half of the curry leaves, half of the sliced onions and garlic till light brown. Take care not to burn the coconut as this will lend a bitter taste to the dish. When almost done, add turmeric powder, chilli powder and coriander powder.
Cool this well and grind to a smooth paste with just enough water.
In another kadai or earthenware pot (if you have, use it), add this ground paste, tamarind extract and 1 cup of water. Season with salt and bring to boil.
Add the prawns and cook till done. Add more water if necessary to get the semi-thick consistency.
Make a tempering with the remaining sliced onions and curry leaves. Add this as a garnish to the dish and stir through just before serving.
Serve hot with steamed white rice.