2 tablespoons unsalted butter
3 tablespoons all-purpose flour
2 cups hot milk
1 teaspoon kosher salt
1/2 teaspoon freshly ground black pepper
Pinch nutmeg
12 ounces Gruyere, grated (5 cups)
1/2 cup freshly grated Parmesan
16 slices white sandwich bread, crusts removed
Dijon mustard
8 ounces baked Virginia ham, sliced but not paper thin