Slice the onion very thinly. Melt the butter in a frying pan, add the onions, season with salt and pepper and cook gently, stirring occasionally, until soft, about 15 minutes. Add the vinegar and reduce until the liquid evaporates, about 1 minute. Remove from the heat.
Sprinkle the pork chops with salt, pepper and sugar. Fry until golden and cooked through, about 5 minutes per side. (If they are lacking in fat, you may need to add a little oil or butter, or both, to the pan to fry them.) Serve with a spoonful of red onion confit on top.