¾ pound penne
1½ cups heavy cream or half-and-half
3 tablespoons finely chopped yellow onion
2 garlic cloves, minced
2 tablespoons all purpose flour
5 ounces goat cheese
3 ounces sharp white cheddar, shredded
½ cup packed freshly grated parmesan
2 tablespoons sour cream
¾ teaspoon lemon zest
1½ teaspoons chopped fresh thyme
Salt and freshly ground pepper
1 extra large egg yolk
Heat oven to 400º. Butter a 10-inch cast-iron skillet. In a large pot of boiling salted water, cook pasta until al dente. Drain and return to pot.
Meanwhile, in a large saucepan, bring heavy cream, onion and garlic to a simmer. Transfer ½ cup of the cream to a medium bowl and gradually whisk in flour; return mixture to saucepan. Set the bowl aside and continue whisking over moderate heat until thickened, about 3 minutes. Remove from heat and whisk in the goat cheese, cheddar and half of the Parmesan until melted. Stir in the sour cream, zest and 1 teaspoon of thyme and season with salt and pepper.
Place the egg yolk in the medium bowl and gradually whisk in ½ cup cheese sauce. Gradually whisk the egg mixture into the saucepan. Pour cheese sauce over pasta and toss to coat evenly. Add pasta to cast-iron skillet and sprinkle with remaining cheese.
Bake for about 25 minutes, until bubbling and golden brown. Sprinkle with remaining thyme. Allow mac and cheese to rest for about 5 minutes (if you can stand it), then serve.