01 - Preheat the oven to 375°F. Grease a 9x13 inch baking dish with butter or nonstick spray.
02 - Cook lasagna noodles according to package instructions until al dente. Drain, rinse gently, and lay flat on a clean surface to prevent sticking.
03 - In a large skillet, heat olive oil over medium heat. Sauté onion and mushrooms for 4 minutes until softened. Add garlic and spinach; cook another 2 minutes until spinach is wilted.
04 - Add shredded chicken to the skillet with the vegetables. Season with dried basil, thyme, black pepper, and salt. Toss to combine evenly and set aside.
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 simmer and cook 5–6 minutes until thickened. Stir in 1/2 cup Parmesan cheese and nutmeg, then remove from heat.
06 - Spread a thin layer of white sauce on the bottom of the prepared baking dish. Layer 3 noodles, half the chicken mixture, 1/3 of the ricotta, 1/3 of the mozzarella, and 1/4 of the remaining sauce. Repeat layers once more. Finish with a final layer of noodles topped with remaining sauce, ricotta, mozzarella, and all remaining Parmesan.
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 - Allow the lasagna to cool for 10–15 minutes before slicing into portions. This resting period helps the layers set for cleaner cuts.