In a large skillet, heat a tablespoon of olive oil over medium heat. Add chopped onions and minced garlic, sautéing until fragrant and soft, about 3-4 minutes. Then, include sliced white mushrooms and cook until they begin to release their moisture, around 5 minutes. Finally, stir in fresh spinach and cook until wilted, just a minute more. Mix in the cooked shredded chicken and set aside.

In a medium saucepan, combine chicken stock and milk over medium heat. Add salt and nutmeg. Bring to a gentle simmer, stirring occasionally. In a small bowl, whisk together flour with water to create a slurry, then gradually whisk into the simmering sauce along with grated Parmesan cheese. Cook until thickened, about 5-7 minutes, and remove from heat.

Preheat oven to 375°F (190°C). In a baking dish, pour half of the white sauce as the base layer. Add a layer of no-boil lasagna noodles, followed by half of the chicken mixture, another layer of white sauce, and mozzarella cheese. Repeat the layers until ingredients are used up, finishing with noodles and sauce.

Cover lasagna with foil and bake for 25 minutes. After removing the foil, bake uncovered for 15 minutes until cheese is golden and bubbling.

Let the lasagna rest for 15-20 minutes before serving. Cut into squares and enjoy.