Preheat oven to 350°F (175°C). Grease a 9×13 inch baking dish.

In a large skillet, heat olive oil over medium heat. Add chopped onion and cook until softened, about 3-4 minutes.

Add minced garlic and mushrooms, cooking for another 3-4 minutes until mushrooms are golden brown.

Stir in baby spinach and cook until wilted. Remove from heat and set aside.

In a medium saucepan, melt butter over medium heat for the white sauce. Stir in flour and cook for 1-2 minutes until bubbly and golden.

Gradually whisk in milk, stirring constantly until smooth and thickened.

Season with salt, pepper, and nutmeg. Simmer for 5-7 minutes until sauce is creamy, stirring occasionally.

In a large bowl, combine shredded chicken, ricotta cheese, mozzarella cheese, Parmesan cheese, dried basil, dried oregano, salt, and pepper.

To assemble the lasagna, spread a thin layer of white sauce on the bottom of the prepared baking dish.

Place 3 lasagna noodles on top of the sauce. Spread half of the chicken and cheese mixture over the noodles.

Spoon half of the mushroom and spinach mixture over the chicken layer.

Pour a third of the white sauce evenly over the layers.

Repeat with another layer of 3 lasagna noodles, remaining chicken and cheese mixture, remaining mushroom and spinach mixture, and half of the remaining white sauce.

Top with the final 3 lasagna noodles and spread the remaining white sauce evenly over the top.

Cover with foil and bake in the preheated oven for 30 minutes.

Remove foil and bake for an additional 10-15 minutes until lasagna is bubbly and top is golden brown.

Let lasagna cool for 10 minutes before slicing. Garnish with fresh parsley if desired before serving.