There are two schools of thought behind the croque monsieur. The simplest, most common version of this popular French snack, which has been showing up on Francophile menus all over town, is not that different from your standard-issue American classic, the grilled ham and cheese. But a true croque monsieur requires a bechamel sauce, which is how Francois Payard does it. His croque monsieur, based on his grandfather's recipe, is layered with jambon de Paris, Swiss Gruyere, and a rich bechamel sauce, encased between slices of toasted white bread, and topped with more bechamel and grated Gruyere. It arrives straight from the oven -- the cheese bubbling and golden-brown, melting over the sides of the bread, and with a rich, creamy center.