Step 1

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.

Step 2

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.

Step 3

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.

Step 4

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.

Step 5

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.