This is a decadent sandwich. It's a decedent vegetarian sandwich, but that doesn't change the mounds of cheese and pot of béchamel... so be careful with this one. It's obviously delicious, but it doesn't come without cost in the calorie department. Instead of ham (for the croque monsieur) we go with grilled zucchini as the main event. Once again you can find the recipe at Serious Eats.
One issue with said recipe is that it calls for nutmeg and mustard but doesn't tell you what to do with them. We added the nutmeg to the béchamel but completely forgot about the mustard. I'm guessing you are supposed to put that on the opposite side as the butter, and based on my experience with mustard and grilled cheese I'd bet the addition is a good one.
Another fun thing about this cooking experience is that we used "shoku pan" from Japonaise Bakery as the bread for our sandwiches. From what I understand it's simply an enriched (i.e. milk is involved) sandwich bread, but it's actually fairly hard to find a legit recipe for some reason. I'd guess it is fundamentally the same thing as pain de mie, but I still found it to be a bit of a revelation because I haven't had bread like this since I stopped eating PBJ's on Wonder Bread at age 10. I think we might be a little too obsessed with "open crumb" and rustic loaves sometimes... enriched white sandwich bread with a tight crumb makes awesome sandwiches and toast, but it's not quite so fashionable. Maybe that should change.