A croque-monsieur is a hot ham-and-cheese sandwich popularized in France after it was first served in a Parisian café in the early 20th century.

Still popular in cafés, bars, bistros and kitchens across France, it consists of thinly sliced Gruyère cheese and lean sliced ham placed on buttered bread, which may be covered in a cheesy Béchamel sauce and broiled in the oven au gratin (or, more rarely, dipped in egg and breadcrumbs before pan-frying in butter). Sometimes the sandwich is topped with a tomato slice and other cheeses and meats can be substituted; add a fried egg, and you've got a croque-madame.

Related: What is a gratin?