Some sandwiches start their life as a Sunday-lunch casserole and only later, after the leftover-management committee meets, end up between two slices of bread. The Plat-en-Sandwich tradition in France is the French answer to the question of what to do with yesterday's stew. It is rarely on a printed menu and almost always on a chalkboard.
The Sandwich Cassoulet is the type case — white beans, duck confit, garlic sausage, slow-cooked together to the texture of pillow-soft pebbles, then spooned cold into a halved baguette with a smear of Dijon. The result eats less like a sandwich and more like a stationary picnic. The Sandwich Choucroute from Alsace does the same trick with sauerkraut, smoked pork, and a thin slice of saucisse. The Sandwich Aligot from Aveyron lands a spoonful of the famously elastic potato-and-tomme-fraîche purée on dense seigle with a few slices of jambon de pays. The Sandwich Tartiflette and the Sandwich Carbonnade Flamande continue the same theme into the Alps and Flanders respectively.
The defining trait of the form is that the sandwich is at least as much about the dish that's been packed into the bread as it is about the bread itself. Done well it's a way to extend a Sunday lunch into Monday's working day; done poorly it's a soggy mess that won't survive the metro. The line is mostly about temperature — these sandwiches are best slightly warm, never hot, never properly cold — and about portion: too much filling and the bread gives up by minute five.