The Croque-Tartiflette is the Savoyard answer to the question of what happens when an Alpine winter casserole climbs into a sandwich. The plat: tartiflette, the gratin of sliced potato, lardons, sweated onion, and a full wheel of Reblochon broken open over the top and baked until the rind melts down into the cream. The sandwich keeps every part of that and stacks it between pain de mie, broils it like a Croque-Monsieur, and serves it hot enough that the cheese is still moving when it hits the plate. It is, accordingly, less a Croque than a casserole that has decided to commute.
The structure pushes the form harder than most Croque variants. Reblochon is a washed-rind cow's-milk cheese from the Aravis range, soft enough that it spreads when warm and pungent enough to register against the lardons. The potatoes have to be parboiled first or they won't tenderize in the time the bread has to crisp, which means this is a sandwich that effectively requires the tartiflette to exist as a separate dish before the sandwich is assembled. The lardons supply salt and fat. The pain de mie keeps its structure because it's been toasted on the broiler side; the underside, in contact with the warm filling, gives up to the cream. Most Croque-Tartiflettes are eaten with a knife and fork in a Savoyard bistro after a half-day on the slopes, which is the use case the dish was built for.
The variations stay close to the Alpine pantry. Some versions trade the Reblochon for Beaufort or Abondance, the harder Savoyard cheeses that hold their shape better under heat but lose the wash-rind funk that defines the dish. Vegetarian versions drop the lardons and lean harder on onion confit. The cross-talk between this and the rest of the Croque family is mostly architectural: the same bread, the same broiler, the same instinct to lift a humble form with real technique. See Croque-Monsieur for the parent and the other regional spin-offs that share its scaffolding.