Start by cooking the lasagna noodles according to package directions if you’re not using oven-ready noodles. Drain them and lay them flat on a clean kitchen towel so they don’t stick together.
Heat olive oil in a large skillet over medium heat. Add the cooked chicken, season lightly with salt, pepper, and Italian seasoning, and warm it through. Set aside.
To make the Alfredo sauce, melt butter in a saucepan over medium heat. Add the minced garlic and cook for about 30 seconds, just until fragrant. Sprinkle in the flour and whisk continuously for 1 minute to cook out the raw flour taste.
Slowly pour in the warm milk while whisking, followed by the heavy cream. Keep whisking until the sauce thickens and becomes smooth. Stir in the Parmesan cheese, salt, pepper, and nutmeg if using. Remove from heat once the sauce is creamy and coats the back of a spoon.
Preheat your oven to 375°F (190°C). Lightly grease your baking dish.
Start layering: spread a thin layer of Alfredo sauce on the bottom, then add lasagna noodles. Top with chicken, ricotta cheese dollops, mozzarella, and more Alfredo sauce. Repeat the layers until everything is used, finishing with sauce and a generous layer of mozzarella.
Cover with foil and bake for 30 minutes. Remove the foil and bake for another 15–20 minutes, until bubbly and lightly golden. Let it rest for at least 15 minutes before slicing.