Put the curry paste and coconut milk in a saucepan. Bruise the lemon grass stalk, by bashing with a rolling pin, and add it to the pan with lime leaves, if using, and sugar. Bring to the boil, then add the monkfish. Simmer gently for 2 minutes, then add the beans and cook for a further 2 minutes or until the fish is cooked through.
Stir in the prawns, fish sauce and lime juice and cook for 2–5 minutes until the prawns turn pink and are cooked through.
Transfer the curry to a warm serving dish and top with coriander sprigs and chilli slices. Serve with plain boiled rice.