Preheat the oven to 375°F. Lightly grease a 9×13-inch baking dish.
Alfredo Sauce
Melt the 4 tablespoons unsalted butter in a large saucepan over medium heat. Add the 1 medium yellow onion and cook until softened, about 5–6 minutes. Stir in the 4 cloves garlic and cook for 30 seconds, until fragrant.
Whisk in the ⅓ cup all-purpose flour and cook for 1 minute. Gradually whisk in the 2 cups whole milk and 1½ cups low-sodium chicken broth until smooth. Stir in the 1 tablespoon Italian seasoning, 1 teaspoon salt, and ½ teaspoon black pepper. Bring to a gentle simmer, whisking frequently, until the sauce has thickened, about 5–7 minutes.
Remove from the heat and stir in the ½ cup freshly grated Parmesan cheese until melted. Fold in the 10 ounces frozen chopped spinach and set aside.
Filling
To a large bowl, add 15 ounces whole milk ricotta cheese, 1 large egg, 3 cups shredded rotisserie chicken, 2 cups shredded mozzarella cheese, ½ cup freshly grated Parmesan cheese, 1 tablespoon Italian seasoning, and ¼ teaspoon black pepper. Stir well to combine.
Assembly
Spread about 1 cup of the white sauce over the bottom of the prepared baking dish. Top with 3 oven-ready lasagna noodles, ⅓ of the ricotta and chicken mixture, ¼ of the remaining white sauce, and about 0.5 cups shredded mozzarella for topping. Repeat the layers two more times. For the final layer, only use the remaining noodles, white sauce, and mozzarella.
Cover the baking dish with greased aluminum foil and bake for 40 minutes. Remove the foil and continue baking for 15–20 minutes, or until the cheese is melted and lightly golden. For a deeper golden top, broil for 2–3 minutes if desired.
Let the lasagna rest for 10–15 minutes before slicing. Garnish with chopped parsley and serve warm.
Notes
Squeeze as much moisture as possible from the spinach to keep the sauce thick and creamy.
Freshly grated Parmesan melts more smoothly than pre-shredded cheese.
Greasing the underside of the foil prevents the melted cheese from sticking during baking.
The chicken doesn’t need to be added to the ricotta mixture. To keep assembly faster, the layers were combined into either the sauce or filling.
Let the chicken lasagna sit for 10-15 minutes before serving to help the layers firm up.
Allow the lasagna to cool completely before storing. Transfer leftovers to an airtight container or cover the baking dish tightly with foil or plastic wrap. Refrigerate for up to 4 days. For longer storage, freeze individual portions or the entire baked lasagna in a freezer-safe container for up to 3 months.
Substitutions
Feel free to use any cooked and shredded chicken.
Fresh spinach may be used instead of frozen. Just roughly chop it and let it wilt in the alfredo sauce.
Use fresh lasagna noodles instead of oven-ready lasagna noodles if you’d rather.