In a large skillet, heat a drizzle of olive oil over medium heat. Add finely chopped onions and minced garlic, sautéing for 2-3 minutes until fragrant and translucent. Stir in sliced white mushrooms along with dried basil, oregano, and red pepper flakes. Cook for an additional 5-7 minutes until the mushrooms are soft. Finally, add fresh spinach and stir until wilted, then mix in the shredded cooked chicken, incorporating everything evenly.

In a saucepan, combine 2 cups of chicken stock and 1 cup of milk over medium heat. Gradually whisk in a slurry of 1/4 cup all-purpose flour mixed with a bit of cold chicken stock until smooth. Bring the mixture to a gentle simmer while continuously stirring for about 5-7 minutes, allowing it to thicken. Once thickened, remove from heat and stir in 3/4 cup grated Parmesan cheese until melted and creamy.

In a 9x13-inch baking dish, spread half of the creamy white sauce on the bottom. Layer in 3 no-boil lasagna noodles, ensuring they are evenly spaced. Spoon half of the spinach and chicken mixture over the noodles, followed by one-third of the remaining sauce. Sprinkle a generous layer of 1 cup of shredded mozzarella cheese on top. Repeat this layering process once more, ending with the last 3 noodles, remaining sauce, and the last cup of mozzarella.

Preheat your oven to 375°F (190°C). Cover the assembled lasagna with aluminum foil to prevent drying out, and place it in the oven. Bake for 25 minutes, then carefully remove the foil and bake for an additional 15 minutes or until the cheese on top is bubbly and golden brown.

Once your lasagna is out of the oven, allow it to rest for 15-20 minutes before slicing. This resting period lets the layers firm up, making serving easier.