Make the Alfredo Sauce: Cook garlic in butter, whisk in flour, then add milk and cream. Simmer until thick, then stir in Parmesan, salt, pepper, and nutmeg.
Mix the Ricotta Filling: Combine ricotta, egg, and seasonings until smooth and creamy.
Cook the Noodles: Boil lasagna noodles until al dente, then drain and lay flat to avoid sticking.
Assemble the Lasagna: Layer noodles, ricotta, chicken, mozzarella, and Alfredo sauce. Repeat. Finish with sauce, mozzarella, and Parmesan on top.
Bake Until Bubbly: Cover and bake at 375°F (190°C) for 25 minutes, uncover and bake 15–20 more until golden. Let rest before serving.