Ingredients   1   and 1/2 Tablespoons unsalted butter 1   and 1/2 Tablespoons all-purpose flour 1   cup   whole milk 8   ounces   Gruyère cheese, coarsely grated 2   ounces   Parmesan cheese, finely grated Salt and freshly ground black pepper, to taste Pinch   freshly grated nutmeg 4   En…