Croque Monsieur

Preheat the oven to 400 F.
Melt the butter over low heat in a small saucepan and add the flour all at once, stirring with a wooden spoon for 2 minutes.
2 tablespoons unsalted butter, 3 tablespoons all-purpose flour
Slowly pour the hot milk into the butter-flour mixture and cook, whisking constantly, until the sauce is thickened.
2 cups hot milk
Off the heat add the salt, pepper, nutmeg, ½ cup grated Gruyere, and the Parmesan and set aside.
1/2 cup grated Gruyere
1 teaspoon kosher salt, ½ teaspoon freshly ground black pepper, pinch of nutmeg, ½ cup freshly grated Parmesan cheese
To toast the bread, place the slices on 2 baking sheets and bake for 5 minutes. Turn each slice and bake for another 2 minutes, until toasted.
16 slices white sandwich bread
Lightly brush half the toasted breads with mustard, add a slice of ham to each, and sprinkle with half the remaining Gruyere. Top with another piece of toasted bread.
1/2 remaining Gruyere
Dijon mustard, 8 ounces baked Virginia ham
Slather the tops with the cheese sauce, sprinkle with the remaining Gruyere, and bake the sandwiches for 5 minutes.
1/2 remaining Gruyere
Turn on the broiler and broil for 3 to 5 minutes, or until the topping is bubbly and lightly browned. Serve hot.