Meanwhile, melt the butter in a large saucepan over medium heat. Whisk in the flour and cook, whisking, 2 mins., then whisk in the milk. Add the bay leaf, nutmeg and 1 t. salt and simmer, whisking occasionally, until thick, 8 mins. Remove the bay leaf and stir in 2 c. cheddar and the gruyere. Stir in the pasta and the reserved cooking water to make a loose sauce.
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