The Sandwich Quiche Lorraine is a slice of quiche eaten between bread, and the curiosity is exactly that: a baked custard tart treated as a sandwich filling. Quiche lorraine is the Lorraine tart of eggs and cream set with lardons in a short pastry shell, baked until the custard is just firm. To make the sandwich a cold or barely-warm wedge is cut, the point trimmed if it will not sit flat, and the slice laid whole into a split crusted loaf, pastry base and custard top together. Nothing is spread and nothing is added; the quiche is already a complete thing. The region is Lorraine.
The craft is the engineering of putting a structured slice inside a loaf, which is stranger than it sounds and is the whole interest of the sandwich. The quiche has its own architecture, a crisp pastry floor, a soft set custard, the salt and smoke of the lardons, and the bread's only job is to be a handle and a contrast rather than a flavor. So the slice has to be set firm enough to hold its shape against the crumb; an underbaked, wobbly quiche slumps and the sandwich collapses. It works best cold or just off room temperature, when the custard is at its most sliceable and the pastry has not gone soft, and it works better in a loaf with a real crust whose chew plays against the smooth custard rather than a soft bread that adds nothing. The lardons supply the seasoning, so the sandwich needs no condiment, though a sharp leaf or a cornichon cuts the richness of eggs and cream the way it cuts any custard. It is portable in the way a slice of cold quiche is already portable: the bread mostly keeps the hands clean.
Variations are the variations of the quiche itself carried into bread, a version with cheese added to the custard, one with onion, one leaner on the cream, the pastry thicker or thinner under the slice. The sandwich changes only as far as the tart does. The Sandwich Quiche Lorraine belongs with the regional dishes the catalog groups under Plat-en-Sandwich, the baked Lorraine and northern preparations that get folded into a loaf. Its specific contribution is the oddest of them: a finished custard tart, pastry and all, used as the filling of a sandwich, held together only by setting the quiche firm and keeping it cold.