The Sandwich Mixte is named for its arithmetic. Mixte means mixed, and in French sandwich shorthand it signals the everyday two-in-one: ham and cheese together in the same baguette, the jambon-fromage that is so common it gets its own one-word label on the counter card. Order a jambon-beurre and you get ham and butter; ask for it mixte and the cheese joins the build. That naming convention is the whole identity of the sandwich. It is not a recipe so much as a standing instruction, understood the same way from one boulangerie to the next: split baguette, butter, jambon blanc, and a firm sliceable cheese, usually a Gruyère, a Comté, or an Emmental, laid in alongside the ham rather than melted over it.
The build works because the two fillings divide the labor. The jambon blanc is pale, mild, gently salted, and on its own with butter it is the clean, restrained jambon-beurre. Add the cheese and you get a second register: the nuttiness and slight resistance of a firm wheel, a saltiness that runs alongside the ham's rather than on top of it, and enough fat that the sandwich carries further into the afternoon. The butter still does its bridging job between the salt of the fillings and the wheat of the crust. The bread has to have a real crust and an open crumb, because nothing here is bound or sauced and the structure comes entirely from the baguette holding two firm fillings against each other. It is the kind of sandwich a counter can build in fifteen seconds and a customer can eat at a desk without a plate, which is most of why it became the default upgrade to the plain ham version.
Variations are mostly swaps of one component while the mixte logic holds. The cheese moves with what is in the case, Emmental in one shop, Comté in another, a young Cantal where the region runs that way; the ham can be a jambon de pays in place of the Paris ham; a few cornichons or a stripe of mustard turn up for those who want an acidic edge. Toast it and press it and it becomes a different sandwich with a different name. The Sandwich Mixte sits next to the Club Sandwich Parisien as the unstacked, two-component everyday answer to the same impulse: more than one good thing between the bread, kept simple enough to make and eat fast.