Gently heat the oil in a large, heavy pan or flameproof casserole dish. Stir in the onions and cook for 5 minutes, then add the green capsicum, celery and ham and cook for a further 4–5 minutes until the vegetables are soft.
Sprinkle with the flour and cook, stirring for 2–3 minutes. Stir in the tomatoes, stock, rice and Cajun seasoning. Bring to the boil, then reduce the heat. Cover and simmer for 15 minutes, or until most of the liquid is absorbed and the rice is almost tender.
Stir in the prawns, cover again and cook gently for 3–4 minutes, or until the prawns have turned pink and are just cooked.
Gently stir in the crabmeat, cover the pan and cook for 1 minute. Remove the pan from the heat and let stand for 3 minutes, then serve sprinkled with chopped coriander. If you like, garnish with coriander sprigs.