Marinade the prawns in salt and turmeric powder while you make the curry. Don't marinate for a long time
Grind all the ingredients for ground masala to a smooth paste using a little water if needed. Keep aside.
In a heavy bottom pan (best to use a mud chatti if you have), heat coconut oil and sautee the onions till they turn golden brown. Add tomatoes and ginger. Cook till they are mushy and leave oil.
Add the ground masala paste, salt, curry leaves and cook for 5-6 minutes till the raw smell is gone.
Add around 2 cups of water, kokum and let the gravy cook for 8-10 minutes.
Add marinated prawns, coconut milk and cook for 3-4 minutes till the prawns are just done. Don't overcook the prawns, they will continue to cook in the heat of the gravy as well.
Serve hot with steamed rice for bliss