01 - Preheat the oven to 375°F. Grease a 9x13 inch baking dish and set aside.
02 - Cook the lasagna noodles according to the package instructions. Drain well and lay them flat on a sheet of parchment paper to prevent sticking.
03 - In a large skillet, heat the olive oil over medium heat. Add the onion and mushrooms, sautéing for 4 minutes until softened. Stir in the garlic and spinach, cooking for an additional 2 minutes until the spinach is fully wilted.
04 - Add the shredded chicken to the skillet with the vegetables. Season with dried basil, thyme, black pepper, and salt. Toss everything together until evenly combined, 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 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 the Parmesan cheese and the nutmeg. Remove from heat.
06 - Spread a thin layer of the white sauce across the bottom of the prepared baking dish. Arrange 3 noodles over the sauce. Layer half of the chicken and vegetable mixture over the noodles, followed by one-third of the ricotta, one-third of the mozzarella, and one-quarter of the remaining white sauce. Repeat with another layer of 3 noodles, the remaining chicken mixture, another third of the ricotta, another third of the mozzarella, and another quarter of the sauce. Finish with a final layer of the remaining noodles, the remaining sauce, the remaining ricotta, the remaining mozzarella, and the remaining 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. Allow the lasagna to rest for 10 to 15 minutes before slicing and serving.