Heat your slow cooker to Low. Peel the outer leaves from the cabbage and discard. Quarter the cabbage, removing the tough stem, then thinly slice. Arrange a layer of the cabbage on the bottom of your slow cooker, then top with some of the onions, apples, zest, mixed spice, sugar and some seasoning. Continue to create layers until you have used up these ingredients. Season.

Pour over the vinegar and cider and dot the butter on top. Cover with a lid and cook for 4-5 hrs until tender. Will keep in the fridge, covered, for up to three days or in the freezer for up to two months. Reheat in a pan or in the microwave.