Peel and finely slice the onion. Peel, core and slice the apples and finely slice the red cabbage. Melt the butter in a pan or casserole over a medium heat (because of the addition of vinegar, do not use any aluminium pans for this recipe). Gently fry the onion in the butter for 3 minutes, then add the apple and cabbage. Cook for 5-6 minutes, until softened a little, then add the sherry vinegar (or red wine vinegar) and the sultanas.
Cover the casserole (pan) with a lid and cook very gently for 1 hour. 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 (about another 1 hour).