This sandwich gives a nod to the ingredients that compose a French mountain dish called tartiflette. The two stars of the dish are an exquisite cave-aged French mountain cheese called Comté and a rather strong cheese called Reblochon. Don't worry if you don't like strong cheeses, though. Brie or Camembert make fine substitutions. Bacon and onions round out the traditional dish, and usually potatoes do, too. But here, the potatoes are part of the bread.