In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add the finely chopped onion and sauté for about 5 minutes, until translucent. Stir in the minced garlic and cook for an additional minute. Next, add the sliced mushrooms, stirring until they turn golden brown, about 6-8 minutes. Finally, mix in the chopped spinach, dried thyme, and oregano until the spinach wilts; season with salt and pepper to taste.

In a medium saucepan over medium heat, melt 4 tablespoons of unsalted butter. Once melted, whisk in 4 tablespoons of all-purpose flour for 1-2 minutes to form a roux. Gradually add 4 cups of whole milk while continuously whisking to avoid lumps. Cook the mixture for 5-7 minutes until it thickens and bubbles. Sprinkle in 1/4 teaspoon of ground nutmeg, along with salt and pepper to taste, ensuring a creamy consistency.

Preheat your oven to 375°F (190°C). Begin by spreading a thin layer of the béchamel sauce on the bottom of a greased 9x13 inch baking dish. Lay down 3 pre-cooked lasagna sheets, then add a portion of the spinach and mushroom filling on top, followed by dollops of ricotta cheese and a sprinkle of shredded mozzarella. Repeat the layering process until all ingredients are used, finishing with a final layer of béchamel and a generous topping of grated Parmesan cheese.

Cover the baking dish with aluminum foil and bake the assembled Creamy Spinach & Mushroom Lasagna for 25 minutes. After this, remove the foil and continue baking for another 15 minutes, or until the top is golden and bubbly. Let the lasagna rest for 10 minutes before slicing to allow the layers to set.