Begin by preheating your oven to 350°F. This ensures your lasagna will cook evenly.
Melt the butter in a large skillet over medium-high heat. Add the minced garlic and chopped onion, cooking until the onion is soft (3-5 minutes).
Add the fresh baby spinach to the skillet, stirring until it wilts down.
Pour in the heavy cream, and stir in the grated Parmesan cheese, ricotta cheese, black pepper, and salt. Add the shredded chicken last. Let the mixture come to a simmer, then remove from heat.
In a 9x13 inch deep baking dish, spread a spoonful of the Alfredo mixture on the bottom. Place about 5 lasagna noodles over the sauce, slightly overlapping.
Spoon a generous amount of the chicken Alfredo mixture over the noodles, spreading it evenly. Sprinkle a third of the shredded mozzarella cheese on top.
Continue layering with noodles, Alfredo mixture, and mozzarella cheese until all ingredients are used, finishing with a layer of cheese on top for the crust.
Cover with foil and bake for about 35 minutes.
Remove the foil and broil for an additional 2-3 minutes.
Let the lasagna sit for about 10 minutes before slicing.