France has more named cheeses than there are days in the year, and a meaningful subset of them belong, properly handled, between the two halves of a baguette. The Sandwich au Brie is the type case — a hunk of slightly-ripe brie de Meaux on a fresh baguette, a thin spread of beurre demi-sel underneath, sometimes a few slices of pear or fig if the cheesemonger is feeling expansive — and it's the sandwich every American foreign-exchange student remembers from a Paris park in May.
The regional variations are a map of French dairy. In the Massif Central you'll find Cantal entre-deux, dense and savoury, layered thick. In Savoie it's a sliver of Reblochon, often with bitter walnuts and a thin layer of jambon de pays. In the Lyon-Beaujolais corridor it's a thick smear of Saint-Marcellin, the soft cow's-milk cheese that wants only good bread and a little salt. The eastern Comté belt offers Comté and Morbier; the Burgundy of Époisses gives you a sandwich that will announce itself across the room; and the Loire Valley's chèvre — Crottin de Chavignol, Selles-sur-Cher, Sainte-Maure — is a separate world of small, intense cheeses that work especially well with a touch of honey and walnut on toasted bread.
The rule that applies across all of them: the cheese was made on a farm, the bread was made that morning, and the sandwich is best eaten within ten minutes of assembly. Anything else and you're eating the idea of the sandwich, not the sandwich itself.