The panino con lonza is a Marche cured-meat sandwich whose defining quality is leanness. Lonza here is the whole pork loin, salted, often rubbed with pepper and a little wine or aromatics, cased and air-dried into a firm, deep-pink cured meat with almost no fat running through it. That is the opposite premise to most of the Italian deli counter, where the fat is half the pleasure: lonza is delicate and dry, and the sandwich is a frame for a clean, restrained slice rather than for richness. The build follows from that, doing as little as possible so a quiet meat is not buried.
The craft is the slice and what is allowed to sit beside a lean cut. Lonza has little fat to keep it supple, so it is cut thin and laid in loose folds rather than stacked in a slab, because air through the slices keeps it from reading dry and tight. The bread is plain and not too assertive, a simple crusted roll or a piece of country pane, since a heavily flavoured loaf would flatten a delicate cured loin rather than carry it. A thin film of butter or a thread of oil is the common concession, used precisely because a very lean meat can want a little fat to bridge it to the crust; beyond that, nothing sharp is added, since the point of the sandwich is the clean, peppered taste of the loin itself.
The variations are modest and largely about that bridge and the bread. There is the plain build, the version with a smear of butter against the leanness, and the one on a soft roll versus a dense country slice. The wider Marche and central-Apennine curing tradition, the soft spreadable ciauscolo and the firmer regional salami, follows its own logic, and each of those deserves its own article rather than being crowded in here.