1. Rinse prawns and remove dark veins, if any. Peel and slice mangoes. Grind coconut coriander, chilli powder and chillies to a paste, adding water.
2. Thinly slice onions. Fry onions in ghee or coconut oil for 2 minutes, then add curry leaves and the coconut paste. Continue to fry until oil begins to rise to the surface. Add the prawns and mango, water, turmeric and salt and simmer on moderately low heat until cooked.
3. Garnish with fresh coriander leaves. For a thicker curry, stir in coconut milk and heat through.