Roast pork and apple sauce is the cold pork sandwich resolved with the sauce that cuts the fat, and the apple sauce is the part that makes it work. Cold roast pork is a mild, faintly sweet meat whose leftover fat sets soft and a little waxy as it cools, coating the palate and reading, on its own between bread, as one heavy register the whole way down. A spread of sharp apple sauce, tart first and only lightly sweetened, cuts straight across that set fat with a clean fruit acid and resets the palate between bites. This is the defining pairing rather than seasoning laid on top: the apple is doing the exact job it does beside the joint on the plate, and it is the part that tells you which roast you are eating. The pork carries the body; the apple sauce carries the relief.
The craft is in the cut and the moisture the sauce supplies. Cold roast pork is best sliced thin and against the grain, because a thick cold slice is dense and pushes the set fat forward while a thin one stays tender and folds to the loaf. Apple sauce is wet, which is its whole virtue here, since the cold meat has lost the running juices it had hot and the sauce puts moisture back along with the acid. That same wetness is the risk: a flood soaks through to a sodden patch within minutes, so it goes on in a measured stripe and the pork is laid as a partial barrier between the sauce and at least one face of the loaf. The bread wants real structure for a dense filling, a sturdy white or a bloomer, with butter to the edges to seal the crumb against the sauce and to bridge the salt of the pork to the wheat.
The variations are mostly a question of the apple's form and what shares the bread with it. A coarse, barely sweetened bramley sauce keeps the acid sharpest; a smoother sweeter apple turns it round and mild; a stripe of sage and onion stuffing alongside adds an aromatic savoury layer; crackling tucked in brings the texture a wet build otherwise lacks. The plain cold-pork slice with no sauce is its own thinner reading. Each deserves its own article rather than being crowded in here.