01 - Preheat the oven to 375°F. Lightly grease a 9x13 inch baking dish with butter or non-stick spray.
02 - Cook lasagna noodles according to package instructions until al dente. Drain thoroughly and lay flat on a clean surface to prevent sticking.
03 - Heat olive oil in a large skillet over medium heat. Sauté the onion and mushrooms for about 4 minutes until softened and fragrant. Add the minced garlic and chopped spinach, cooking for another 2 minutes until the spinach is fully wilted.
04 - Add the shredded chicken to the skillet with the vegetables. Season with dried basil, dried thyme, black pepper, and salt. Stir until the chicken is evenly coated with the seasonings, then remove from heat and set aside.
05 - In a medium saucepan, melt the butter over medium heat. Whisk in the flour and cook for 1 minute to form a roux. Gradually pour in the whole milk while whisking constantly to prevent lumps. Bring the mixture to a gentle simmer and cook for 5 to 6 minutes, stirring frequently, until the sauce thickens enough to coat the back of a spoon. Stir in 1/2 cup of the Parmesan cheese and the nutmeg, then remove from heat.
06 - Spread a thin layer of white sauce across the bottom of the prepared baking dish. Lay down 3 noodles to cover the sauce. Spread half of the chicken and vegetable mixture over the noodles. Dollop 1/3 of the ricotta cheese evenly across the layer, followed by 1/3 of the mozzarella. Pour about 1/4 of the remaining white sauce over the cheese. Repeat with another layer of 3 noodles, the remaining chicken mixture, another 1/3 of the ricotta, another 1/3 of the mozzarella, and more sauce. Finish with a final layer of the remaining noodles, the rest of the sauce, the remaining ricotta, the remaining mozzarella, and top with the rest of the Parmesan cheese.
07 - Cover the baking dish tightly with aluminum foil and bake for 30 minutes. The foil traps moisture and allows the layers to heat through evenly.
08 - Remove the foil and continue baking for an additional 15 minutes, or until the top is golden brown and the cheese is bubbly and melted. Allow the lasagna to rest for 10 to 15 minutes before slicing and serving so the layers set properly.