Toast the desiccated coconut in a baking tray in the oven for about 10 minutes at 160°C, or until golden brown and fragrant. Set aside to cool. Heat the oil in a saucepan and gently sautee the red onion, garlic and chilli until soft. Take off the heat and mix in the coconut milk. Boil the prawns and leave to cool. Gently mix all the ingredients together by hand, adding lime juice and fish sauce to taste. Serve immediately.