Put the milk and macaroni in a medium saucepan. Bring the milk to a boil over medium heat, stirring frequently to keep the macaroni from clumping, then cook, stirring frequently, until the macaroni is tender and the milk has thickened to the consistency of heavy cream, 4 to 5 minutes. Remove the saucepan from the heat, add the Cheddar, mozzarella, cream cheese, butter, mustard, cayenne, nutmeg and 1 1/2 teaspoons salt, and stir until smooth, thick and creamy. Serve hot. (The dish will thicken as it cools; thin it out with a little hot water if desired.)
Be sure to buy blocks of cheese and shred it yourself. The preshredded cheese that comes in bags is often tossed with starchy cellulose, which can give this mac 'n' cheese a clumpy texture.
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