As anyone who has visited Paris knows, a very popular lunchtime meal at cafes and smaller eateries in the City of Light—and indeed throughout all of France—is the croque monsieur, a toasted ham and cheese sandwich that is one of the true staples of simple French cuisine.

But how did it become such a famous French dish and land on the menus of countless casual restaurants?

No one is absolutely certain, but one version of the story suggests that the sandwich was created entirely by accident when French workers left their lunch pails too close to a hot radiator, and the heat toasted the bread and melted the cheese in their sandwiches.

Another croque origin story is more elaborate, and helps explain how the dish made it onto the menus of French eateries. This tale of the sandwich’s creation dates back to the year 1901 and a Paris brasserie on the Boulevard des Capucines.  Having run out of baguettes for the restaurant’s sandwich of the day, the chef took a loaf of pain de mie (similar to American sandwich bread), sliced it, placed ham and cheese between the slices and baked it to crispiness.

This tale even includes how the name croque monsieur came about. The name is derived from the crispy bread of the sandwich (from the French verb croquer, which means “to bite,” and from a casual comment from the brasserie’s chef about the origins of the ham in the sandwich. When asked by a customer about the meat, the chef reportedly gestured toward another customer—likely the neighborhood butcher—and replied “C’est la viande de monsieur (It’s that guy’s meat).” And voila–le croque monsieur.

However the croquet was invented, the sandwich was referred to by name for the first time on a Parisian cafe menu in 1910 and its first mention in print appears in Proust’s 1918 work In Search of Lost Time.

The main ingredient in the sandwich is thinly sliced ham, typically topped with Emmental or Gruyere cheese. The sandwich is then buttered and grilled to crispiness. The croque monsieur is then usually topped with béchamel sauce (and perhaps a sprinkling of grated cheese) and placed under the broiler for a moment or two. (Many cafes and restaurants nowadays take shortcuts in preparing the sandwich, and just assemble all the ingredients and shove the entire concoction under the broiler to cook.)

When topped with a poached or lightly fried egg, the sandwich is called a croque madame, so named because the egg is said to resemble a woman’s hat. That particular sandwich name dates to about the year 1960.

Most cafes, bistros and brasseries offer only the standard croque monsieur and croque madame, but there are variations that can be had at eateries specializing in sandwich making or those attempting to upgrade their croque offerings, including by many up-and-coming chefs anxious to create their own classic sandwiches. Sometimes certain ingredients are added to give the sandwich an international flair, such as a Scandinavian flavor or Spanish taste.

Common variations include:

• croque poulet that replaces the ham with grilled chicken;
• croque provencal that include a slice of tomato;
• croque auvergnat made with bleu d’Auvergne cheese;
• croque gagnet made with Gouda cheese and andouille sausage;
• croque norvegien that uses smoked salmon in place of ham;
• croque tartiflette that includes slices of potatoes and Reblochon cheese;
• croque bolognese (also commonly called the croque boum-boum) made with Bolognese sauce;
• croque senor that includes salsa;
• and croque Hawaiian that includes a slice of grilled pineapple.