Preheat oven to 180C/350°F Pour the stock into a saucepan and add the lemon grass and half the kaffir lime leaves. Bring to the boil and simmer, covered for ten minutes.
Heat the oil in an oven proof casserole dish and add the onion cooking until soft but not coloured. Add the curry paste and cook for another minute. Add the rice and stir until coated. Strain the stock into the rice and add the coconut cream. Cover and bake for 15 minutes.
Remove from the oven, stir well and bake for another 15 minutes.
Add the prawns and if dry add another 1/2 cup of stock or water. Bake for another 15 minutes until the prawns are cooked and the rice tender.