Prepare the Béchamel Sauce: Melt 4 tablespoons of butter in a medium saucepan over medium heat. Whisk in 4 tablespoons of flour and cook until golden, then add 4 cups of milk, whisking until thickened. Season with salt and pepper, then remove from heat.

Sauté the Vegetables: Heat 2 tablespoons of olive oil in a large skillet over medium-high heat. Add 8 ounces of sliced mushrooms and sauté until browned. Add 4 cups of fresh spinach and cook until wilted, then set aside.

Layer the Lasagna: Preheat oven to 375°F (190°C). Spread a thin layer of béchamel sauce in a 9x13 inch baking dish. Layer with lasagna noodles, sautéed mushrooms and spinach, and cheese. Repeat layers ending with noodles topped with béchamel and cheese.

Bake the Lasagna: Cover with foil and bake for 25 minutes, then remove foil and bake for an additional 15-20 minutes until golden.

Rest Before Serving: Allow the lasagna to rest for 10-15 minutes before slicing.