While no cheese sauce can get away with the "skinny" moniker, this one isn't terribly sinful because it's milk-based rather than cream-based. The addition of gooey Piemontese cheese makes it silky, rich, and smooth. I suggest La Tur, a soft-ripened cheese made from a blend of sheep, cow, and goat's milk. If you cannot find it, look for Taleggio, which is from nearby Lombardia and makes a fine substitute. If these suggestions may as well be Greek (Italian) to you, and your local grocer doesn't carry either of these cheeses, look for something with a soft, creamy consistency, such as a triple-creme Brie.
Bring a large pot of salted water to a boil. Add the pasta and cook according to the package's directions.
Meanwhile, melt the butter in a medium saucepan over medium heat. Whisk in the flour and stir with a wooden spoon for 3 minutes, to eliminate the raw flour taste from the finished sauce. Add the milk in a slow, steady stream, whisking vigorously to avoid clumping. When the milk mixture begins to simmer, turn the heat to medium-low and cook for 5 minutes, stirring occasionally. Add the cheese all at once, along with the nutmeg and salt. Reduce the heat to low and stir constantly to melt the cheese into the sauce. The sauce is ready when there are no more clumps of cheese, but it can be held over the lowest heat setting on your stove for 15 minutes.
Drain the pasta, toss the sauce with the pasta and serve immediately.