For Grinding Fry the coconut on a skillet without adding any oil till the coconut turns brown. Then dry fry the coriander cumin and mustard red chillies(both) and garam masala. Then fry the small onion for few seconds. Put all the above ingredients in a blender along with the tomatoes,ginger and garlic pieces,and tamarind. Then add the turmeric and salt (1 tsp). and 1 green chili. Grind it to a thick paste. Add very little water to keep it coarse. Vinegar Add 1 tbsp of vinegar and dilute it with water For Marination Make a gash on the prawns on the back and clean to remove the black thread. Marinate the prawns with salt. After a few minutes when the prawns absorbs the salt. Add some portion of the above grinded masala to the prawns and keep it for 1/2 hour. Cooking Pour oil. Then add the large onion and add salt (small pinch). Add the ginger garlic chili paste and fry till the smell of garlic mellows down.Then add the remaining part of the masala (grinded) and fry it for five minutes till the masala oozes the oil out. Then add the marinated prawns( do not boil it separately). and then add water as per your consistency. Keep on stirring. Then half the way add the vinegar diluted with water. Add salt as per your taste and health.Continue till the prawns are cooked and your kitchen is filled with the aroma. Finish it off with a garnish of some freshly cut coriander leaves.