Heat one tablespoon of the oil in a large frying pan. Fry the prawns for about five minutes until pink then remove to a bowl and set aside.
Heat the rest of the oil in the same pan and add the onion; cook for about five minutes until softened then add the peppers and fry for five more minutes. Add the garlic, ginger, lemongrass and chilli pastes and cook, stirring, for a minute then add the coconut milk, peanut butter, fish sauce and sugar.
Allow to simmer for five minutes then add the prawns, coriander, basil, lime juice and some salt and pepper to taste. Simmer for a few more minutes until the prawns are warmed through then serve with white rice.