Bring a large pot of salted water to a boil. Cook the pasta for 3 minutes less than the package directions instruct. Drain and set aside in the same pot.
Preheat your over to 360 degrees F.
Place a large skillet over medium heat. Add the oil and while that's heating up season the chicken pieces all over with salt and pepper. Add the chicken to the skillet and cook for 4-5 minutes, until almost cooked through. (It'll finish cooking in the oven.) Transfer to the pot with the pasta.
To the same skillet add the butter. Once melted, add the garlic and cook for 1 minute until fragrant. Add the flour and stir into the butter.
Add the half-n-half a little at a time to the roux while whisking until it's all been added, making sure there are no lumps. Simmer for 2-3 minutes until thickened.
Add the nutmeg, some pepper, and Parmesan to the sauce. Whisk until the cheese has melted in. Season with salt to taste.
Pour the sauce into the pot with the pasta and chicken along with 1 cup of mozzarella. Stir to combine.
Pour the mixture into a 9-inch x 13-inch baking dish and spread into an even layer. Top with the remaining cup of mozzarella.
Bake for 15-20 minutes until the cheese on top is melted and the sauce is bubbling. Remove from the oven, sprinkle the top with parsley, and serve warm.
Notes
You can add vegetables like spinach or broccoli. These fit very well. Blanche the broccoli before adding. You can also cook the broccoli with the pasta, just add to the pasta the last 5 minutes of cooking time.
Crispy bacon adds a nice flavor too. Cook your bacon, crumble it, and add it along with the rendered fat to the sauce.