In a heavy-based pan, heat the oil and add the curry paste. Fry for 2 mins, add the sugar and turmeric, then stir well. Splash in the fish sauce, then the coconut milk and pineapple. Bring to the boil. Add the lime leaves or juice, chilli and lemongrass. Simmer for 3 mins, throw in the prawns and simmer for 5 mins more until the prawns change colour.