Croques Monsieur 1 tablespoon unsalted butter 1 1/2 tablespoons all-purpose flour 1 cups milk 1/2 teaspoon salt 2 1/2 Gruyere or other swiss, grated 1/4 cup freshly grated Parmesan 8 slices white sandwich bread, crusts removed 8 oz sliced ham 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 Pour the milk into the butter–flour mixture in slow stream, whisking Cook and whisk until the sauce is thickened