In a large frying pan, heat oil over medium heat. Add onion and sauté till softened.
Add coconut to the onions and sauté for a few minutes. Mix in turmeric, cayenne pepper and salt. Add prawns and stir. Reduce heat to low and cook for 15 minutes. Serve hot.
To crack open a fresh coconut, place it on a tea towel or newspaper on the floor. Then hit it firmly with a hammer across the middle. As soon as it starts to crack, put it over a bowl so you can drain the water. Break the coconut into smaller pieces to make the flesh easier to remove. Use a knife to remove the coconut by sliding it between the white flesh and the shell.