Alfredo Sauce: To a 4 quart heavy bottomed saucepan, melt the butter over medium heat. Add the garlic cooking just until fragrant, about 1 minute.
To the pot add the flour whisking constantly until the flour has been absorbed.
Gradually whisk in the half-and-half, 1 tsp salt, black pepper and nutmeg. Bring to a gentle boil whisking constantly then lower the heat to a simmer. Let cook for 5 minutes whisking occasionally.
After 5 minutes, to the sauce add 1 cup grated Parmesan Reggiano cheese. Mix to combine and remove from heat stirring until smooth.
Ricotta-Spinach Mixture: In a medium bowl, whisk together ricotta, egg, Italian seasoning, onion powder, 1/2 tsp salt, red pepper flakes or black pepper and 1/3 cup grated Parmesan Reggiano cheese. Add the spinach mixing to combine then set aside.
Preheat oven to 350°F. Spray a a 2 1/4-inch deep 13x9-inch baking dish with cooking spray.
Assemble Lasagna: Divide lasagna noodles into thirds. Spread a thin layer of sauce (about 1/2 cup) on the bottom of the dish. Arrange one layer of noodles in the bottom of the pan. Spread the noodles with 1/2 of the ricotta-spinach filling then top with 2 cups chopped chicken. Drizzle with 1/3 of the sauce. Sprinkle with 1 1/2 cups shredded mozzarella cheese.
Repeat layers starting with noodles, remaining ricotta-spinach mixture, 2 cups chopped chicken and drizzle with 1/3 sauce. Sprinkle with 1 1/2 cups shredded mozzarella cheese.
Top with the final layer of noodles and alfredo sauce. Sprinkle with remaining 1 1/2 cups mozzarella cheese and remaining grated Parmesan-Reggiano cheese.
Spray a large piece of aluminum foil with cooking spray and cover the dish. Bake in preheated oven until hot and the noodles are tender, about 55-60 minutes. (Spraying the foil prevents the cheese from sticking).
Remove the foil and broil at the end of baking to brown the top further.
Let stand for 5-10 minutes, then slice and serve garnished with Italian parsley.