Preheat a large skillet over medium-high heat, adding the unsalted butter, minced garlic, and chopped onion. Sauté until the onion turns translucent.
Stir in the baby spinach until it wilts down, blending well with the other ingredients.
Mix in the heavy cream, grated Parmesan, ricotta cheese, black pepper, salt, and the shredded chicken. Bring the mixture to a gentle simmer, then take it off the heat.
Spread a thin layer of the chicken Alfredo mixture at the bottom of a 9×13-inch baking dish. Arrange 5 lasagna noodles over the mixture.
Add a generous portion of the chicken Alfredo mixture over the noodles, then sprinkle with one-third of the shredded mozzarella.
Repeat the layers of noodles, Alfredo mixture, and mozzarella until all the ingredients are used, ensuring that the topmost layer is mozzarella.
Cover the dish with foil and bake in a preheated oven at 350°F for 30 to 35 minutes.
Remove the foil and broil for 2 to 3 minutes until the cheese on top becomes golden brown.
Allow the lasagna to rest for 10 to 15 minutes before slicing and serving.