The Sandwich Tablier de Sapeur is a Lyonnais bouchon dish folded into bread, and it is a curiosity worth understanding. Tablier de sapeur, the sapper's apron, is a Lyon specialty of beef tripe, traditionally marinated, then breaded and fried until the crust is gold and the inside stays tender. The name comes from the shape of the breaded piece, broad and flat like a leather work apron. Putting it in a split baguette is the move that turns a knife-and-fork bouchon plate into something you can carry: a panko-crisp slab of tripe, a sharp sauce, and a sturdy loaf.
The craft is in protecting the crust and cutting the richness, which is the whole reason this is hard. Breaded and fried tripe lives and dies by its coating: crisp and just-fried it is the point of the dish, steamed soft inside a wrapped sandwich it is a disappointment. So the build wants assembly close to the fryer, a bread with enough crust to add a second texture rather than competing for the same one, and the dish's traditional partner, the sauce gribiche or a sharp rémoulade, the chopped-egg-and-mustard-and-pickle sauce that exists precisely to cut the fried richness. That sauce is not a condiment choice here, it is the counterweight the dish is built around on the plate, and the sandwich keeps it for the same reason. A leaf or a few cornichons reinforce the same job. It is best within a few minutes of frying, while the crust still cracks and the bread is still firm, because both go the wrong way as they sit.
Variations stay inside the Lyonnais frame rather than leaving it. The constant arguments are about the sauce: gribiche for the classic chopped-egg sharpness, a tartare-style rémoulade for a creamier read, a plain mustard for a more direct bite. Some builds add a leaf for freshness, others keep it strictly tripe and sauce and bread. The frame is fixed: a breaded fried slab of tripe, a sharp egg-and-pickle sauce to cut it, a crust to set against the coating. It belongs with the roast and beef sandwiches the catalog groups under Baguette Rôti / Bœuf, and its specific contribution is a bouchon fried specialty made portable, where the entire problem is keeping the crust crisp and the richness in check between two halves of bread.