Twenty-four hours before you plan to serve, 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, 1.5 Cups of the grated cheese, 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. Stir gently every few hours to ensure distribution. The pasta will absorb the cream and expand.
When ready to cook, allow the mixture to sit at room temperature for about an hour. (I recommend using gloved hands or a spoon to gently break up any clumped pasta) Meanwhile, preheat the oven to 400 degrees.
Butter a 2-quart gratin dish, transfer the mixture to the dish and spread it evenly.
Add the 1.5 cups cheese 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.