Preheat the slow cooker if necessary; see the manufacturer's instructions. Put the pork chops into the base of the slow cooker pot and spoon over the vinegar and sugar. Sprinkle the onions on top, then add the apples.
Put the cornflour and mustard in a small bowl and mix with a little cold water to make a smooth paste, then gradually stir in the boiling stock until smooth. Pour over the pork. Cover with the lid and cook on high for 30 minutes. Reduce the heat and cook on low for 6½–7½ hours, or set to auto for 7–8 hours, until the pork is cooked through and tender.
Transfer the pork to serving plates, stir the sauce and spoon over the chops. Sprinkle with chopped chives, if liked, and serve with mash and Brussel sprouts.