Ham, Gruyère, and Parmesan Sauce for Asparagus

Ingredients

• 1 tablespoon minced onion
• 1 ½ tablespoons unsalted butter
• 2 tablespoons all-purpose flour
• 1 ½ cups milk
• ¼ cup grated Gruyére
• ¼ cup freshly grated Parmesan, or to taste
• ¹/₃ cup finely chopped cooked ham

1. In a saucepan cook the onion in the butter over moderately low heat, stirring, until it is softened, stir in the flour, and cook the roux, stirring, for 3 minutes. Remove the pan from the heat and add the milk, scalded, in a stream, whisking until the mixture is thick and smooth. Simmer the sa...

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