Creating this White Lasagna with Spinach and Mushrooms might seem daunting, but it can be straightforward if you follow these steps:

1. Preheat Oven: Preheat your oven to 375°F (190°C).
2. Cook Noodles: Boil the lasagna noodles according to the package instructions until al dente. Drain and set aside.
3. Sauté Vegetables: In a skillet, heat olive oil over medium heat. Add minced garlic and sliced mushrooms, cooking until mushrooms are tender. Stir in chopped spinach and cook until wilted. Season with salt, pepper, and oregano.
4. Prepare Béchamel Sauce: If you’re making your own béchamel sauce, melt 4 tablespoons of butter in a saucepan, whisk in 4 tablespoons of flour, and gradually add 3 cups of milk, stirring until the sauce thickens. Add nutmeg, salt, and pepper for flavor.
5. Layer Ingredients: In a 9×13-inch baking dish, spread a thin layer of béchamel sauce at the bottom. Layer 4 lasagna noodles on the bottom, followed by half of the ricotta cheese, half of the sautéed vegetables, and a layer of béchamel. Add a layer of mozzarella cheese and sprinkle with Parmesan.
6. Repeat Layers: Repeat the layers, starting with lasagna noodles, then remaining ricotta, vegetables, béchamel, mozzarella, and Parmesan. Finish with a final layer of noodles topped with remaining béchamel and cheeses.
7. Bake: Cover the dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 20 minutes, or until the top is golden and bubbling.
8. Let it Rest: Once done, remove from the oven and let it sit for 15 minutes to firm up before slicing and serving.

These straightforward steps will guide you in creating an incredible White Lasagna with Spinach and Mushrooms that will surely impress your guests!