The Sandwich Flammekueche takes an Alsatian tart and folds it into a handheld. A flammekueche, the tarte flambée of Alsace, is a thin sheet of bread dough rolled out almost cracker-flat, spread with fromage blanc or crème fraîche, scattered with thinly sliced raw onion and lardons of smoked pork, then baked fast and very hot until the rim chars and the cream blisters. To eat it as a sandwich you take that finished sheet and fold it: the crisp base becomes its own bread, the creamy onion-and-lardon face becomes the filling, and the whole thing closes into a hot parcel you hold in one hand.
The craft is a balance of crisp and soft that only works in a tight window. Straight from the oven the base is brittle and shattering, the cream is hot and loose, the onion has gone sweet and translucent, the lardons have given up smoke and salt into the crème. Fold it then and it eats clean. Let it sit and the steam off the topping softens the base from underneath until the fold goes limp and the parcel will not hold its shape. There is no separate loaf doing structural work here, so the dough has to be rolled thin enough to crackle yet baked sturdy enough to survive being bent in half while loaded with hot cream. The lardons are the anchor flavor, the smoked pork salt against the dairy fat and the onion's sweetness; the whole thing reads rich and lightly singed.
This is an Alsatian dish first, the kind of thing eaten in rounds straight off the board, and the folded sandwich is simply the version you can carry away from the table.
Variations stay inside the Alsatian register rather than wandering off. The gratinée adds cheese over the top; a sweeter close swaps the savory face for apple and cinnamon. Each is a recognizable adjustment of the same thin-baked sheet. The Sandwich Flammekueche belongs with the regional dishes folded into bread the catalog groups under Plat-en-Sandwich. Its specific contribution there is that the bread and the filling are baked as one sheet: there is no second slice, only a thin tart hot enough to fold before it goes soft.