01 - Preheat the oven to 375°F. Grease a 9x13-inch baking dish with butter or non-stick spray.
02 - Cook the lasagna noodles according to package instructions until al dente. Drain and lay flat on a clean surface to prevent sticking. Skip this step if using no-boil noodles.
03 - Heat olive oil in a large skillet over medium heat. Sauté the finely chopped onion until softened, about 3 minutes. Add the minced garlic and cook for an additional 1 minute until fragrant.
04 - Stir the spinach into the skillet and cook until wilted. Transfer the mixture to a 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 ground 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. Arrange 3 noodles over the sauce. Layer half of the chicken-spinach filling evenly over the noodles, followed by one-third of the mozzarella and one-third of the remaining white sauce. Repeat with another 3 noodles, the remaining chicken-spinach filling, another third of the mozzarella, and another third of the sauce. Finish with the last 3 noodles, the remaining sauce, and top with the remaining 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 sauce is bubbling. Let the lasagna rest for 10 to 15 minutes before slicing and serving.