This goes brilliantly with almost any winter dish you care to mention. It takes minutes to prepare, then two hours of gentle cooking - you need do nothing except have a glass of wine and relax.
Melt the butter in a stainless steel or flameproof casserole over a medium heat (an aluminium pan will not work for this).
Fry the onion in the butter for three minutes, then add the apple and cabbage.
Cook for five minutes or so, until softened, then add the sultanas and the sherry vinegar.
Place a lid on the pan and cook for one hour, until all the vegetables are just tender.
Season to taste with salt and freshly ground black pepper and replace the lid. Continue to cook until the cabbage is tender and all the liquid has evaporated. This dish will keep for several days, covered, in the fridge. Simply heat it up again when ready to serve.