The saucisson sandwich is what French shop owners give their kids for lunch and what French train passengers reach for at the kiosk. A length of baguette is split, occasionally buttered, and stacked with slices of saucisson sec — usually the Lyonnais rosette, sometimes the Auvergne-style with garlic and black pepper, sometimes the Corsican lonzu when you're past Marseille.
The sandwich's structure is: bread that pulls, sausage that bites back. The rosette is the type case — a thick, slow-cured pork sausage from Lyon, marbled with fat the way good prosciutto is marbled, sliced thicker than you'd slice prosciutto and laid in shingles. Butter is not strictly mandatory but it bridges the salt of the sausage and the wheat of the crust in a way that the dry-on-dry version cannot. Cornichons are optional; a glass of cool Beaujolais Villages is a separate matter.
The regional cousins extend the same idea into different animals and curing traditions. The Sandwich Andouille — sliced tripe sausage from Vire or Guémené, smoky and unmistakable — has its devotees and its skeptics. The Sandwich Rillettes spreads cooked-down pork (or duck, or rabbit) onto the same bread, and the Sandwich Boudin Blanc replaces dry curing with the cream-and-egg refinement of the white sausage. The Corsican figatellu and lonzu, the Alsatian knack and cervelas, the Lyonnais Jésus, the Savoyard diots, the Maghrebi-French merguez — each turns the sandwich into a passport stamp for whichever charcuterie hometown the cook came from.