Croque-monsieur is, very simply, a grilled ham and cheese; the easy addition of a fried egg on top transforms it into a croque-madame. There are many different versions of this classic French fast food, including smothering the final product with cheesy mornay sauce. Here I've opted for the plain version, but as with any sandwich, you should feel free to explore all the different variations until you create your own perfect version.