HOW TO MAKE A CROQUE-MONSIEUR
I also like to add some Béchamel sauce inside - I swirl it around with the mustard. If you didn't have Bechamel you could probably use a little real mayonnaise and it wouldn't hurt anything. Or, just use mustard only. Your choice.
Add a slice or 2 of thin sliced ham. It's not traditional to pile up a big stack of ham. Just a thin layer is all you need. On top of the ham, add a thin slice of Gruyere or some shredded Gruyere, and top with the other piece of bread.
Now, you can do this 2 ways. Assemble the sandwich and cook it in the oven, turning as it browns, but I just find it so much easier to grill it in a pan with a little butter, and then finish under the broiler.
Because it was dinner, I served it with a salad dressed with olive oil and lemon, and made some tomato soup.