01 - Preheat the oven to 375°F. Grease a 9x13 inch baking dish and set aside.
02 - Cook lasagna noodles according to package instructions. Drain thoroughly and set aside on a clean surface or parchment paper to prevent sticking.
03 - In a large skillet, heat olive oil over medium heat. Sauté the onion and mushrooms for 4 minutes until softened. Add the garlic and spinach, cooking for another 2 minutes until the spinach is fully wilted.
04 - Add the shredded chicken to the skillet with the sautéed vegetables. Season with dried basil, thyme, black pepper, and salt. Stir to combine evenly, 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 milk while whisking constantly until smooth. Bring to a simmer and cook for 5 to 6 minutes until the sauce thickens. 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, followed by half the chicken mixture, one-third of the ricotta, one-third of the mozzarella, and one-quarter of the remaining sauce. Repeat the layers once more, then finish with a final layer of noodles topped with the remaining sauce, ricotta, mozzarella, and Parmesan cheese.
07 - Cover the baking dish tightly with aluminum foil and bake for 30 minutes.
08 - Remove the foil and continue baking for an additional 15 minutes, or until the top is golden brown and the sauce is bubbly. Allow the lasagna to rest for 10 to 15 minutes before slicing and serving.