Mutton chops were popular in Victorian and Edwardian times. They have more hard marbled fat than lamb chops because they come from older sheep. The age of the animal also means the chops need to cook longer, but the extra fat keeps the meat moist. Lamb, in contrast, is usually served pink, which is difficult to do when making this recipe, as the chops are cooked twice, on a griddle and under a broiler.
If your regular butcher counter doesn’t sell mutton, it’s worth the effort to see