Preheat the oven to 375°F and lightly grease a 3-quart baking dish with cooking spray.
In a large saucepan set over medium heat, melt the butter. Introduce the garlic and sauté for 30 seconds. Stir in the flour, whisking continuously, and cook for an additional minute.
Gradually incorporate the milk and cream, maintaining a steady whisk until the mixture thickens slightly.
Blend in the Parmesan cheese and 1/2 cup of the mozzarella, seasoning with salt and pepper. Stir until the cheese has fully melted into the sauce.
In a large mixing bowl, combine the cooked penne and diced chicken. Pour the sauce over the mixture and toss gently to coat all components evenly.
Transfer the coated pasta and chicken mixture into the prepared baking dish. Distribute the remaining mozzarella over the surface.
Bake, uncovered, for 20 minutes or until the cheese is melted and lightly golden, and the dish is bubbling throughout.
Finish with a sprinkling of chopped parsley before serving.