Clean the prawns and pat dry. Marinate with salt and turmeric powder and allow to sit till the other ingredients are ready.
To prepare the spice powder, add all the whole spices to a small pan and roast on low heat till aromatic. Cool and then grind to a fine powder. Since it makes extra, store the remaining for later use in an airtight container.
To make the coconut paste, heat oil in the same pan and add the sliced onions. Sauté till the onions are light brown and then add the grated coconut. Continue to sauté on low heat till the coconut turns light brown as well. Cool and grind to a paste with a little bit of water.
Heat the remaining oil in another deep bottom pan. Add half of the curry leaves followed by the onions; sauté till the onions turn light brown.
Next add the tomatoes and sauté till softened. Then add the coconut pasta and mix well to combine. Cook till the mixture comes together and oil starts to leave the sides of the pan.
Next add 2 tbsp of the spice blend along with the chilli powder followed by the prawns. Mix well and cook till the prawns are half cooked.
Then add sugar, vinegar and adjust seasoning. Mix and continue to cook till the prawns are almost done. Then add the coconut milk and simmer gently for another minute.
Turn off heat and garnish with remaining curry leaves and coriander leaves.
Serve warm
