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 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 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, stirring constantly to form a smooth roux. Slowly pour in the whole milk and heavy cream while whisking continuously. Continue cooking and whisking for 5 to 7 minutes until the sauce thickens enough to coat the back of a spoon. Stir in the ground nutmeg, the 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 blended.
06 - Spread a thin layer of the white sauce across the bottom of the prepared baking dish. Arrange 3 lasagna noodles over the sauce. Spread half of the chicken-spinach filling evenly over the noodles. Sprinkle with one-third of the mozzarella cheese and drizzle with one-third of the remaining white sauce. Repeat with another layer of 3 noodles, the remaining chicken-spinach filling, another third of the mozzarella, and another third of the sauce. Top with the final 3 noodles, the remaining sauce, the remaining mozzarella, and the remaining 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 to allow the layers to set.