Cooler highland air does the work in this one, drying a mountain ham off the slopes of the Tarn that the sandwich takes its name from. Jambon de Lacaune is a dry-cured pork leg, salt-rubbed and matured in the cooler air of the highlands of the Tarn, and the curing gives it a firm, lean grain, a pronounced salt, and a deep, slightly rustic flavor distinct from the softer hams of the lowlands. The components are minimal: a fresh baguette, a thick layer of barely-salted butter, and thin slices of the cured ham. The defining element is the ham's mountain character, and the butter is the part that keeps it from turning severe.
The craft is in the slicing and the fat balance, the same logic every dry-cured ham sandwich runs on but tuned to this ham's leanness. Lacaune ham is firm and assertive; cut thick it chews hard and the salt sharpens, so it has to come off the knife thin enough to drape and fold, releasing its depth in a bite rather than a wall. Curing has pulled the moisture out, so the sandwich carries none of the wetness a poached ham would, which means the butter is structural: spread thick, it cushions the salt, carries the lean meat's flavor, and stops the crumb reading as dry. The bread needs a firm crust to match the density of the ham. Because there is little moisture to soak inward, the sandwich holds up through a short wait, but it is still best within a few minutes of being built, while the butter is cool and the crust has bite.
Variations are the rest of the dry-cured shelf, each ham tuned by its own region and salt level: a southwestern leg, a generic jambon sec, a parsleyed terrine that leaves the form behind. Those deserve their own qualifiers and their own articles rather than being crowded in here. This sandwich is one regional reading of the canonical Jambon-Beurre: the same baguette and butter, a lean Tarn mountain ham standing in for the pale poached one, with the knife and the fat carrying the difference.