(slightly adapted from Barefoot Contessa)
2 tablespoons unsalted butter
3 tablespoons all-purpose flour
2 cups hot milk
1 teaspoon kosher salt
1/2 teaspoon freshly ground black pepper
1/8 teaspoon freshly ground nutmeg
2 cups grated Gruyere cheese
1/4 cup freshly grated Parmigiano-Reggiano cheese
8 slices white sandwich bread, crusts removed
4 ounces baked Virginia ham, sliced
Preheat the oven to 400 degrees.
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. Slowly pour the hot milk into the butter-flour mixture and cook, whisking constantly, until the sauce is thickened. Off the heat add the salt, pepper, nutmeg, 1 1/2 cup grated Gruyere, and the Parmigiano-Reggiano and set aside.
To toast the bread, place the slices on a baking sheet and bake for 5 minutes. Turn each slice and bake for another 2 minutes, until toasted.
Lightly brush each piece of toasted bread with the mustard. To 4 of the pieces, add a slice of ham and sprinkle with the remaining Gruyere. Top with the other pieces of toasted bread. Slather the tops with the cheese sauce, sprinkle with the remaining Gruyere and bake the sandwiches for 5 minutes. Turn on the broiler and broil for 3 to 5 minutes, or until the topping is bubbly and lightly browned. Serve hot.