Roast pork and crackling is the cold pork sandwich resolved by texture rather than by sauce, and the crackling is the whole argument. Cold roast pork between bread is a soft problem: a mild, faintly sweet meat with set fat, laid on soft bread, gives a sandwich that is uniformly yielding from the first bite to the last and reads as one register because nothing in it resists the teeth. A shard of crackling, the skin roasted until it has blistered hard and glassy, is the answer. It snaps, it shatters, and it puts a loud crisp note against the soft meat and the soft crumb so that the sandwich has a top and bottom of texture instead of one flat middle. The pork carries the flavour; the crackling carries the point of eating it this way at all.
The craft is keeping the crackling crisp until it is eaten. Crackling holds its snap only while it is dry, and the enemies are steam and time: warm meat below it, a wet sauce beside it, or a long wait under cling film all soften it back to leather, which is the one failure this sandwich cannot survive. So the pork is cold and fully so, any sauce is kept thin and to one side, and the crackling is added in clean shards at the last moment rather than built in early. The shards are broken to a size that bites cleanly rather than dragging the whole piece out of the sandwich. The bread wants structure to hold a dense filling, a sturdy white or a bloomer, buttered to the edges to bridge the salt to the wheat and to keep the crumb firm under the weight.
The variations are mostly a question of what moisture, if any, is allowed in beside the crackling. A thin smear of sharp apple sauce reintroduces the fat-cutting acid without drowning the snap; sage and onion stuffing adds an aromatic savoury layer at the cost of a little crispness; the plain pork-and-crackling pairing keeps the texture argument cleanest. The hot hog roast roll with crackling pulled straight off the spit belongs to its own tradition. Each deserves its own article rather than being crowded in here.