Heat the oil in a large pan and cook the carrot, celery, chilli and ¾ of the red onion with some seasoning for 10-15 minutes or until soft. Add the garlic, spices and oregano, and cook for 1 minute, then stir in the tomato purée and cook for another minute. Tip in the sweet potato, chopped tomatoes and 350ml of water, and simmer gently for 25-30 minutes or until the sweet potato is completely tender. Add the prawns and a little seasoning, and simmer for 5 minutes until they have turned pink and are cooked through. Stir the remaining red onion through the avocado and spoon over the soup, with the coriander. Serve with lime wedges.