Preheat oven to 350 degrees F. Spray an 8x8-inch baking dish with nonstick cooking spray and set aside.

Melt butter in a large saucepan over medium heat. Add chopped onion and cook for about 4 minutes, stirring occasionally, until soft and translucent.

Stir in minced garlic and cook for 30 seconds until fragrant. Do not let it brown.

Whisk in the flour and stir for 1 minute to cook out the raw taste. Slowly pour in the heavy cream while whisking. Bring to a gentle simmer and cook, stirring often, until the sauce thickens and coats the back of a spoon, about 3 to 5 minutes.

Remove from heat. Stir in Parmesan, cream cheese, chicken, spinach, salt, and pepper. Keep stirring until the spinach wilts and everything is smooth and combined. If the sauce is too thick, add a small splash of cream to loosen it.

In a small bowl, whisk together the ricotta and egg until smooth and creamy.

Spread about 1/4 cup of Alfredo sauce on the bottom of the prepared baking dish. Lay 3 cooked lasagna noodles on top.

Gently spread 1/4 cup of the ricotta mixture over the noodles. Add 1/2 cup of sauce, then sprinkle 2/3 cup shredded mozzarella. Repeat this layering two more times for a total of 3 layers.

Pour any remaining sauce over the top and sprinkle generously with extra mozzarella cheese.

Cover tightly with aluminum foil and bake for 38 to 42 minutes, until the sauce is bubbling around the edges.

Remove foil and broil for 2 to 3 minutes, watching closely, until the cheese on top is golden with crispy edges.

Let the lasagna rest for 18 minutes before slicing. This resting time is essential for clean, neat slices.