Heat 1 tbsp olive oil in a large skillet over medium-high heat. Sauté sliced cremini mushrooms for 5-6 minutes until browned. Remove and set aside. In the same skillet, add remaining oil and minced garlic, sautéing for 30 seconds. Toss in fresh spinach, cooking for 1-2 minutes until completely wilted. Drain all excess liquid thoroughly.

Melt butter in a saucepan over medium heat. Whisk in flour for 1-2 minutes. Gradually pour in warmed milk, whisking constantly until smooth. Simmer for 4-5 minutes until thickened. Remove from heat; stir in Parmesan, nutmeg, salt, and pepper.

Preheat oven to 375°F (190°C). Grease a 9x13-inch baking dish. Spread a thin layer of sauce on the bottom, followed by a layer of noodles. Distribute a portion of chicken, spinach, mushrooms, white sauce, and mozzarella. Repeat layers, finishing with noodles, remaining sauce, mozzarella, and a few decorative mushroom slices on top.

Cover loosely with foil and bake for 25 minutes. Remove foil and bake uncovered for 15 minutes until golden and bubbling (broil for 1-2 minutes if desired). Let rest for 15 minutes. Garnish with fresh basil, chopped parsley, and cracked black pepper before slicing.