Easy Croque Monsieur

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1. 1Step 1: Melt butter in a medium saucepan over medium heat. Whisk in flour and cook, stirring constantly, for about 3 minutes. Gradually add milk, stirring well until the mixture is smooth. Cook, stirring, until sauce is thickened.
2. 2Step 2: Season with a little bit of salt and pepper. Remove from heat and whisk in mustard and nutmeg. Set aside while you make the sandwiches, or make the sauce up to 1 week in advance.
3. 3Step 3: Preheat oven to 425° F. Line a baking tray with parchement paper and set aside.
4. 4Step 4: Spread each bread slice with a layer of béchamel, spreading it all the way to the edges. Place 4 slices of bread, béchamel side up, on prepared baking sheet.
5. 5Step 5: Top each with a piece of ham, a handful of gruyere, and a sprinkle of parmesan cheese. Place remaining slices of bread on top, béchamel side up, then top with remaining gruyere and parmesan cheese.
6. 6Step 6: Bake at 425 degrees F for about 5-6 minutes, until cheese is melted, and then turn the oven to broil and broil until the cheese on top is lightly golden, 2-4 minutes.