Season the flour with salt and pepper on a plate. Coat the oxtail with the flour.
Heat the oil in a flameproof casserole and fry the oxtail in batches until browned on all sides, lifting out with a slotted spoon on to a plate. Add the onions and celery to the casserole and fry gently for 5 minutes. Add the star anise, ginger and any flour left over from coating, and cook, stirring, for 1 minute.
Blend in the stock, tomatoes, orange rind and juice and soy sauce. Return the oxtail to the pan and bring to a simmer, stirring. Cover and cook in a preheated oven, 150°C (300°F), Gas Mark 2, for about 3½ hours or until the meat falls easily from the bone.
Fold in the coriander and season to taste with salt and pepper. Serve garnished with extra coriander.