Twenty-four hours before you plan to serve the gratin, bring a large pot of water to a boil. Add 2 tablespoons salt and the macaroni and cook for exactly 4 minutes. Drain and set aside.
Meanwhile, in a medium (10-inch) bowl, combine the cream, nutmeg, 1 cup of the Gruyere, ½ cup of the Cheddar, 2 teaspoons salt, and 1 teaspoon pepper. When the pasta is done, stir it into the cream mixture, cover the bowl with plastic wrap, and refrigerate for a full 24 hours. The pasta will absorb the cream and expand.
When ready to cook, allow the mixture to sit at room temperature for about an hour. (You can also microwave it for 4 minutes.) Preheat the oven to 400 degrees.
Butter a 2-quart gratin dish, transfer the mixture to the dish and spread it evenly. Combine the remaining 1 cup Gruyere and ½ cup Cheddar and sprinkle it evenly on top. Combine the bread crumbs and 2 tablespoons butter and sprinkle evenly on top. Bake for 20 to 25 minutes, rotating halfway through, until golden brown and crusty. Serve hot.
Note: To make 1½ cups of fresh bread crumbs, remove and discard the crusts from 5 slices of white bread. Dice the remaining bread and put it in the bowl of a food processor fitted with the steel blade and process until you have fine crumbs.
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