Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a large frying pan, add the onion and fry for 5 minutes or until softened.
Stir in the garlic, celery and carrot, then mix in the tomatoes, stock, rice, herbs and chillies. Add a little salt and pepper and bring to the boil. Pour into the slow cooker pot, cover with the lid and cook on high for 3–4 hours or until the vegetables and rice are tender.
Stir the soup, then add the okra and dressed brown crab meat. Replace the lid and cook, still on high, for 20–30 minutes. Ladle the soup into bowls, top with the flaked white crab meat, if liked, and serve with warm crusty bread.