01 - Preheat the oven to 375°F. Grease a 9x13-inch baking dish with butter or nonstick spray.
02 - Cook the lasagna noodles according to package instructions until al dente. Drain and lay flat on a sheet of parchment paper to prevent sticking. Skip this step if using no-boil noodles.
03 - Heat olive oil in a large skillet over medium heat. Add the finely chopped onion and sauté until softened, about 3 minutes. Add the minced garlic and cook for 1 additional minute until fragrant.
04 - Stir the spinach into the skillet and cook until wilted. Transfer the mixture to a large mixing bowl and combine with the shredded chicken. Season with salt, pepper, and half of the dried Italian herbs. Set aside.
05 - In a medium saucepan, melt the butter over medium heat. Whisk in the flour and cook for 2 minutes to form a smooth roux. Slowly pour in the whole milk and heavy cream, whisking constantly until the sauce thickens, about 5 to 7 minutes. Stir in the nutmeg, remaining Italian herbs, and a pinch of salt and pepper. Remove from heat and fold in the ricotta cheese and half of the Parmesan until smooth and well combined.
06 - Spread a thin layer of the white sauce across the bottom of the prepared baking dish. Layer with 3 noodles, half of the chicken-spinach filling, one-third of the mozzarella, and one-third of the remaining sauce. Repeat the layers once more. Finish with a final layer of noodles topped with the remaining sauce, mozzarella, and Parmesan cheese.
07 - Cover the dish tightly with aluminum foil and bake for 35 minutes. Remove the foil and continue baking for 15 to 20 minutes until the top is golden brown and the edges are bubbling. Let the lasagna rest for 10 to 15 minutes before slicing and serving.