01 - Preheat oven to 375°F. Grease a 9x13 inch baking dish with butter or non-stick spray.
02 - Bring a large pot of salted water to a boil. Cook lasagna noodles according to package directions until al dente. Drain, rinse gently under warm water, and lay flat on a clean surface to prevent sticking.
03 - Heat olive oil in a large skillet over medium heat. Add onion and mushrooms, sautéing for 4 minutes until softened and fragrant. Add garlic and spinach, continuing to cook for 2 minutes until the spinach is fully wilted.
04 - Add shredded chicken to the sautéed vegetables. Season with dried basil, thyme, black pepper, and salt. Stir thoroughly to combine and set aside off the heat.
05 - In a medium saucepan, melt butter 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. Remove from heat.
06 - Spread a thin layer of white sauce across the bottom of the prepared baking dish. Arrange 3 noodles over the sauce. Layer half the chicken mixture evenly over the noodles, followed by 1/3 of the ricotta, 1/3 of the mozzarella, and 1/4 of the remaining sauce. Repeat with another layer of 3 noodles, the remaining chicken mixture, half the remaining ricotta, half the remaining mozzarella, and another quarter of the sauce. Finish with a final layer of 3 noodles, the remaining sauce, the rest of the ricotta, the remaining mozzarella, and the remaining 1/2 cup of Parmesan on top.
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 edges are bubbling.
09 - Remove from the oven and let rest for 10 to 15 minutes before slicing. This allows the layers to set and makes serving easier.