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 clean surface 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 an additional 1 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. Toss to distribute evenly.
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 to prevent lumps. Continue cooking and stirring until the sauce thickens, 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 creamy.
06 - Spread a thin layer of the white sauce across the bottom of the prepared baking dish. Layer 3 noodles over the sauce. Spread half of the chicken-spinach filling evenly over the noodles. Sprinkle one-third of the mozzarella over the filling, then pour one-third of the remaining white sauce on top. Repeat the layering process once more with 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 the remaining mozzarella and Parmesan on top.
07 - Cover the baking 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 to allow the layers to set.