Bring a large pot of water to a rolling boil. Add 1 tablespoon kosher salt, then stir in the penne. Cook 1 minute less than package directions for al dente texture, usually 10-11 minutes. Reserve 1 cup pasta water, then drain and set aside. Do not rinse.

In a large skillet over medium heat, melt the butter until foamy. Add the minced garlic and cook 30 seconds until fragrant but not browned. Sprinkle in the flour and whisk constantly for 1 minute to cook out the raw taste. The mixture should look like wet sand and smell toasty.

Slowly pour in the milk while whisking vigorously to prevent lumps. Add the cream, then increase heat to medium-high. Cook, whisking often, until the sauce bubbles and thickens enough to coat the back of a spoon, about 6-8 minutes. You should be able to draw a line through it with your finger.

Remove from heat. Stir in the grated Parmesan, nutmeg, and black pepper until the cheese melts completely and the sauce is silky smooth. If it seems too thick, thin with a splash of reserved pasta water.

Add the drained penne directly to the skillet with the sauce. Toss vigorously for 1 minute over low heat, adding pasta water 2 tablespoons at a time as needed, until every piece is coated and the sauce clings rather than pools. The pasta should look glossy, not soupy.

Divide among warm bowls immediately. Top with extra grated Parmesan and chopped parsley. Serve hot with crusty bread for scooping up every last drop.