It’s easy to think of a croque madame as little more than a grilled ham and cheese sandwich, but it may also be the most indulgent grilled cheese you can get your hands on.

In its basic form, this classic dish is a hot sandwich made with toasted bread, Gruyère cheese, béchamel sauce and lean ham, all topped with a fried egg. That’s usually exactly how it’s presented, but there is plenty of room for customization, including choice of bread, cheese selection, use of local or specialty ham and addition of condiments such as hot sauce or chutney.

According to market research firm Datassential, croque madame’s presence on U.S. restaurant menus has grown by nearly one-third over the past four years. One-fifth of Americans know about this Flavor of the Week and nearly 10% have tried it.

Click through the slideshow to find out more about this French dish, which is a favorite especially among Millennials.