Preheat the oven to 200°C fan/220°C/Gas 7. Deeply score through the rind of the chop with a very sharp knife at 2cm intervals all the way along. Peel the apple and cut off two sides as close to the core as possible. (Eat the smaller two uncut sides.) Splash the apple pieces all over with cider vinegar, then scatter a little sugar over them followed by some salt.
Melt the butter in an ovenproof frying pan over a medium–low heat. When it starts to foam, lay in the sage leaves without overlapping. Cook, undisturbed, for a minute or so on both sides, or until crisp, and then transfer to a piece of kitchen paper.
Turn up the heat a little and season the chop well. Lay the apple pieces flat on the chopping board and, keeping them intact at one end, cut them into 5mm slices. Keeping each sliced half together, as if in one piece, transfer them to the frying pan with the chop. Push the apple slices a bit, so that they lean to one side. This will help them catch some good colour. Cook the apple and chop for 2 minutes or so, or until the pork is deep and richly browned, then turn the chop on to its uncooked side, spoon some butter over the apple pieces and put the whole frying pan in the oven for 7–8 minutes, or until the pork is cooked but retains the faintest pinkness.
Put the chop and apples on a plate and pour over a little of the butter from the frying pan. Pile the crispy sage on top and eat with a glass of good cider.