Preheat the slow cooker if necessary; see the manufacturer's instructions. Cut each celery heart in half lengthways, then rinse under the cold tap to remove any traces of dirt. Drain and put into the slow cooker pot.
Mix the remaining ingredients together and pour over the celery. Cover with the lid and cook on high for 4–5 hours or until the celery is tender. If you would prefer a thicker sauce, pour off the liquid from the slow cooker pot into a saucepan and boil rapidly for 4–5 minutes to reduce. Pour back over the celery and serve as an accompaniment to roast chicken, pork or duck.