Heat the oil in a large saucepan, add the oxtail pieces and fry until browned on one side. Turn the oxtail pieces over and add the onion, stirring until browned on all sides. Stir in the carrots, celery, potatoes and herbs and cook for 2–3 more minutes.
Pour in the stock and ale, then add the mustard, Worcestershire sauce, tomato purée and butter beans. Season well with salt and pepper and bring to the boil, stirring. Half cover the pan and simmer gently for 4 hours.
Lift the oxtail and herbs out of the pan with a slotted spoon. Discard the herbs and cut the meat off the oxtail bones, discarding any fat. Return the meat to the pan, reheat then taste and adjust the seasoning if needed. Ladle into bowls, sprinkle with chopped parsley and serve with crusty bread.