Heat the oven and boil the pasta: Preheat the oven to 350°F and grease a 9x13-inch baking dish. Bring a large pot of salted water to a boil and cook the penne 2 minutes less than the package says for al dente. Drain and set aside.

Sear the chicken: Toss the chicken with the Italian seasoning, garlic powder, 1/2 teaspoon of the salt, and 1/4 teaspoon of the pepper. Heat the olive oil in a large skillet over medium-high heat and cook the chicken for 6-8 minutes, stirring occasionally, until browned and a thermometer reads 165°F. Transfer to a plate.

Start the roux: Melt the butter in the same skillet over medium heat. Add the minced garlic and cook for 30 seconds until fragrant, then sprinkle in the flour and whisk constantly for 1 minute to cook off the raw flour taste.

Build the alfredo sauce: Slowly whisk in the milk and heavy cream, scraping up any browned bits from the bottom of the pan. Simmer for 4-5 minutes, whisking often, until the sauce thickens enough to coat the back of a spoon. Pull it off the heat and stir in 1 cup of the parmesan, the nutmeg, and the remaining 1/2 teaspoon salt and 1/4 teaspoon pepper until smooth.

Combine everything: In a large bowl, toss the drained penne and the seared chicken with the alfredo sauce until every noodle is coated.

Layer the chicken alfredo bake: Spread half the pasta mixture into the prepared dish and scatter 1 cup of the mozzarella over it. Add the rest of the pasta, then top with the remaining 1 cup mozzarella and the remaining 1/2 cup parmesan.

Bake: Bake uncovered at 350°F for 25-30 minutes, until the cheese on top is golden in spots and the sauce bubbles around the edges of the dish.

Rest and garnish: Let the casserole sit for 5 minutes so the sauce sets up enough to cut clean squares, then sprinkle with fresh parsley and serve.