Preheat oven to 375°F. Lightly grease a 13x9-inch baking dish with butter or nonstick spray.
In a large saucepan over medium heat, melt butter. Add garlic and cook for 30 seconds, just until fragrant. Whisk in flour and cook for 1–2 minutes.
Slowly whisk in heavy cream and milk, stirring constantly until smooth. Simmer for 5–7 minutes, stirring often, until thickened.
Remove from heat and stir in 1 1/2 cups of the Parmesan, 1 teaspoon salt, 1/2 teaspoon pepper, and nutmeg until smooth.
Toss the cooked chicken with salt, pepper, and garlic powder.
Spread a thin layer of Alfredo sauce in the bottom of the prepared baking dish. Make 2 layers as follows: lasagna noodles, alfredo sauce, chicken, mozzarella, and remaining Parmesan cheese.
Cover loosely with foil and bake for 30 minutes. Remove foil and bake an additional 10–15 minutes, until bubbly and lightly golden.
Let the lasagna rest for 10–15 minutes before slicing. Garnish with fresh parsley or basil if desired.