Croque Monsieur is essentially a toasted cheese and ham sandwich. Put a fried egg on top and you've got a Croque Madame (the egg is supposed to resemble a lady's hat). What makes the difference between a toasted cheese and ham sandwich and a Croque Monsieur is the cheese – in a Croque Monsieur it comes in the form of a creamy cheese sauce. And boy, does this make a difference!
My version of Croque Madame uses the bread as a muffin cup to contain the delicious cheese sauce and egg. Great as a snack, or have it with a green salad and fries, as they serve it in French cafés.
For the Mornay (cheese) sauce:
To Make the Sauce: Melt the butter in a pan over a medium heat. Add the flour and beat hard until you have a smooth paste. Take off the heat and leave to cool for 2 minutes, then gradually add the milk, whisking constantly. Place the pan back over a medium heat, add the mustard and nutmeg, and simmer gently for 10 minutes, whisking frequently to stop the sauce burning on the bottom of the pan. Once the sauce thickens and has the consistency of a thick tomato sauce, take it off the heat. Add the cheese (keep a little for the garnish) and taste for seasoning. If the sauce is too thick, add a little more milk. If it's lumpy, pass it through a sieve.
To assemble, preheat the oven to 350°F. Flatten the slices of bread with a rolling pin, then brush each slice on both sides with melted butter. Line a 6-cup muffin tin with the slices of bread, press¬ing them in with the bottom of a small glass. Divide the ham between the muffin cups followed by the eggs (if the egg seems too big, pour a little of the white away before using). Put 2 tablespoons cheese sauce on top of each egg, then sprinkle with a little cheese and pepper. Bake for 15-20 minutes, depending on how runny you like your eggs. Serve immediately.
From The Little Paris Kitchen: 120 Simple but Classic French Recipes, by Rachel Khoo. Chronicle Books, copyright 2013.