01 - Preheat the oven to 375°F. Lightly grease a 9×13-inch baking dish with butter or nonstick spray.
02 - Bring a large pot of salted water to a boil. Cook the lasagna noodles according to package directions until al dente. Drain carefully and lay flat on a clean sheet of parchment paper to prevent sticking. Skip this step if using no-boil noodles.
03 - Heat the olive oil in a large skillet over medium heat. Add the chopped onion and cook, stirring occasionally, until softened and translucent, about 3 minutes. Add the minced garlic and cook for 1 minute longer until fragrant.
04 - Add the fresh spinach to the skillet and cook, tossing gently, until completely wilted. Transfer the vegetable mixture to a large mixing bowl and combine with the shredded chicken. Season with salt, pepper, and half of the dried Italian herb blend. Toss to distribute evenly and set aside.
05 - In a medium saucepan, melt the butter over medium heat. Sprinkle in the flour and whisk constantly for 2 minutes to form a pale blond roux. Slowly pour in the whole milk and heavy cream in a steady stream, whisking vigorously to prevent lumps. Continue cooking and whisking until the sauce thickens enough to coat the back of a spoon, 5 to 7 minutes. Stir in the 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 creamy.
06 - Spread a thin layer of the white sauce across the bottom of the prepared baking dish. Arrange 3 noodles side by side over the sauce. Spread half of the chicken-spinach filling evenly over the noodles. Scatter one-third of the mozzarella over the filling, then drizzle with one-third of the remaining white sauce. Repeat with a second layer of 3 noodles, the remaining chicken-spinach filling, another third of the mozzarella, and another third of the sauce. Finish with the last 3 noodles on top, the remaining sauce, and 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 more, until the top is deeply golden and the edges are bubbling. Let the lasagna rest for 10 to 15 minutes at room temperature before slicing and serving.