In a large skillet, heat 2 tablespoons of oil over medium-high heat. Add 1 diced onion, 2 minced garlic cloves, and 8 ounces of sliced white mushrooms. Sauté for about 5 minutes until translucent. Stir in 4 cups of fresh spinach and cook until wilted, about 2 minutes. Add 2 cups of shredded cooked chicken, mixing well before setting aside.

In a medium saucepan, combine 2 cups of chicken stock, 2 cups of milk, a pinch of nutmeg, and salt to taste. Simmer over medium heat. Gradually whisk in a slurry made from ¼ cup of flour mixed with ½ cup of water, stirring until thickened, about 5 minutes. Remove from heat and fold in 1 cup of grated Parmesan cheese.

Preheat oven to 375°F (190°C). In a 9x13-inch baking dish, pour half of the sauce on the bottom. Layer noodles, half of the chicken mixture, and more sauce. Sprinkle 2 cups of mozzarella cheese on top. Repeat layering, finishing with noodles and remaining sauce.

Cover with aluminum foil and bake for 25 minutes. Uncover and bake for an additional 15 minutes, until cheese is bubbly and golden.

Let the lasagna rest for 15-20 minutes after baking for easier slicing. Cut into squares and serve warm.