Slice the onions very thinly. Melt the butter in a frying pan, add the onion, 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 a minute. Remove from the heat.
Season the pork chops with salt, pepper and sugar. Fry until golden and cooked through, about five 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.