Pain Perdu Salé takes the technique of French toast and points it away from sugar. Stale bread, usually a slice of pain de mie or a thick cut of brioche, is soaked in a savoury custard of beaten egg and milk seasoned with salt and pepper rather than vanilla and sugar, then pan-fried in butter until the outside sets golden and the inside stays soft and set. That custard-soaked slab is the sandwich base. Between two of them, or folded over once, goes a savoury filling: ham and a melting cheese, sautéed mushrooms, sometimes a slice of soft cheese left to go molten in the residual heat. The defining move is that the bread is cooked, not just filled, and the egg is part of the bread rather than a separate layer.
The construction is built around what soaking does to tired bread. Stale crumb that would crumble dry instead drinks the custard, firms up in the pan, and turns into a surface that is crisp at the edges and almost flan-soft in the middle. That texture is the point: it is richer and more yielding than toasted bread, and it carries a warm filling without going to mush because the egg has already given the slice its structure. The cheese melts against a hot surface, the whole thing is eaten with a knife and fork off a plate, and it does not travel. This is a kitchen sandwich, made from what is going stale, eaten warm and soon, the savoury cousin of a dessert that exists for the same reason: bread that needed using.
Variations follow whatever the savoury custard is asked to carry. The plainest is ham and Gruyère between two custarded slices, finished under heat until the cheese pulls. A mushroom-and-shallot version leans earthier and skips the meat entirely. Some cooks fold a single soaked slice around the filling like an omelette rather than stacking two, which keeps it lighter. The brioche base pushes it richer; a firmer country loaf keeps it closer to savoury. The constant is the method: stale bread, savoury custard, the pan, a warm filling, eaten promptly. It sits within the wider tradition of sandwiches built on breads other than the baguette, covered by Pain Garni & Non-Baguette Breads, where the bread is treated as the part of the sandwich that does the work.