1Position a rack in the center of the oven and preheat the oven to 350°F. Lightly butter a 9 by 13-inch baking dish.
2Bring a large pot of salted water to a boil over high heat. Add the cavatappi and cook, stirring occasionally, until the pasta is just short of al dente, about 2 minutes less than the recommended cooking time on the package directions. The pasta will be cooked again in the oven, so it is important not to overcook it. Drain well and return to the cooking pot.
3Meanwhile, melt the butter in a medium saucepan over medium heat. Whisk in the flour. Reduce the heat to low and let bubble without browning for 1 minute. Gradually whisk in the hot milk and bring to a simmer. Cook, whisking often, until the sauce is lightly thickened and no raw flour taste remains, about 10 minutes. Remove from the heat. Add the cheese and let it soften in the sauce for about 3 minutes. Whisk until the cheese melts and the sauce is smooth. Season the sauce to taste with the salt and pepper and stir in the nutmeg. Mix the pasta and sauce together in the pasta cooking pot. Spread in the prepared baking dish.
4To prepare the bread crumbs: Mix the bread crumbs and Parmigiano well in a small bowl. Add the melted butter and mix with your fingertips until combined. Sprinkle evenly over the pasta.
5Bake until the pasta is lightly browned, 20 to 30 minutes. Let stand for 5 minutes, then serve hot.
TIP Boschetto al tartufo is our preferred cheese for this dish because it is rind- less (meaning no waste) and has plenty of real truffles. It is sold at well-stocked cheese stores. Supermarkets sometimes carry other kinds of truffled cheeses, but these often are made with artificially flavored truffle oil. Most truffle oil has never been near a real truffle, and we don’t use it.