Shell, devein and clean the prawns. Set them aside. If making stock, clean the shells and any heads thoroughly, then boil them in a little water until the liquid reduces by half. Strain the stock before adding it to the curry.
Cut the raw mango and okra. Boil them in water until almost cooked.
While the vegetables cook, roast the coriander seeds, cumin seeds, fenugreek seeds, mustard seeds, peppercorns and Byadagi chillies.
Grind the roasted spices with the coconut, turmeric, tamarind, chopped half-onion and garlic to a smooth paste, using a little water.
Add the coconut masala to the vegetables, followed by the strained prawn stock if using. Add salt and bring the curry to a boil. Check the seasoning.
Add the prawns and cook for around 3 minutes, until opaque and cooked through. Switch off the heat once they are cooked to avoid overcooking them.
In a separate pan, brown the large chopped onion in coconut oil. Pour it over the curry and mix gently.
Serve with steaming hot rice and happala.