01 - Preheat oven to 375°F. Lightly grease a 9x13 inch baking dish with butter or nonstick spray.
02 - Bring a large pot of salted water to a boil. Cook lasagna noodles according to package directions until al dente. Drain, lay flat on parchment paper, and set aside.
03 - Heat olive oil in a large skillet over medium heat. Add onion and mushrooms, sautéing for 4 minutes until softened. Stir in garlic and spinach, cooking for another 2 minutes until the spinach is fully wilted.
04 - Add shredded chicken to the skillet with the vegetables. Season with dried basil, thyme, black pepper, and salt. Toss everything together until well combined, then remove from heat and set aside.
05 - Melt butter in a medium saucepan over medium heat. Whisk in flour and cook for 1 minute to form a roux. Gradually pour in milk while whisking constantly until smooth. Bring to a gentle simmer and cook for 5 to 6 minutes until the sauce thickens enough to coat the back of a spoon. Stir in 1/2 cup of Parmesan cheese and 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, then distribute half the chicken mixture evenly. Dollop 1/3 of the ricotta, scatter 1/3 of the mozzarella, and drizzle 1/4 of the remaining sauce. Repeat with another layer of 3 noodles, the remaining chicken filling, another third of ricotta and mozzarella, and more sauce. Finish with a final layer of noodles, the remaining sauce, ricotta, mozzarella, and the rest of the Parmesan on top.
07 - Cover the dish tightly with aluminum foil and bake for 30 minutes. Remove the foil and continue baking for 15 more minutes until the top is golden brown and the edges are bubbling.
08 - Remove from the oven and let the lasagna rest for 10 to 15 minutes before slicing. This allows the layers to set and makes serving easier.