Heat the oil in a saucepan, add the onion and fry gently for 5 minutes until softened and just beginning to brown. Stir in the carrot, celery and red pepper and fry for a few more minutes. Mix in the tomatoes, thyme, chilli and tomato purée, then pour in the stock. Add the rice, season with salt and pepper and bring to the boil.
Cover and simmer for 20 minutes, stirring occasionally. Halve any very large mussels, then stir into the soup with the remaining seafood, canned crab and okra. Cover and simmer for 5 minutes, then taste and adjust the seasoning if needed. Ladle into bowls and sprinkle with a few thyme leaves, if liked. Serve with crusty bread.