When the leg on the slicer is salt-cured and air-dried but wears no regional badge, a counter just calls it jambon sec and this is the sandwich that follows. The term simply means dried ham: a pork leg rubbed with salt and matured until it firms, darkens, and concentrates, with no claim to a specific place. The components are a fresh baguette, a thick layer of barely-salted butter, and thin slices of whatever cured ham the charcutier has hung. The defining quality is the salt-and-air concentration of dried meat, and the butter is what makes that concentration sit comfortably in a sandwich.
The craft is the same discipline every dry-cured build runs on, applied without a regional ham's particular profile to lean on. Dried ham is salty and firm; cut thick it turns chewy and the salt becomes the only thing you taste, so it has to be sliced thin enough to drape and fold, yielding to the bite rather than forming a wall. Curing has driven the moisture out, so the sandwich brings none of the wetness a boiled ham would, and the butter is therefore structural rather than decorative: spread thick and kept cool, it cushions the salt, carries the meat's nuttiness, and keeps the crumb from reading as dry. The bread needs a firm crust to stand against the firm ham. With little moisture to soak inward, the sandwich tolerates a short wait, though it is still best within a few minutes of assembly, while the butter holds and the crust still bites.
The variations are precisely the named hams this generic version stands in for: a southwestern leg, a Tarn mountain ham, and the rest of the regional dry-cured shelf, each with its own salt level and depth. Those carry their own qualifiers and their own articles rather than being crowded in here. The Sandwich au Jambon Sec is the unbadged baseline of the dry-cured side of the canonical Jambon-Beurre: the same baguette and butter, an air-dried leg in place of the pale poached one, named plainly because no single region is being claimed.