Heat a large skillet over medium-high heat and add a drizzle of olive oil. Sauté 8 ounces of sliced mushrooms until golden brown, about 5-7 minutes. Stir in 6 cups of fresh spinach, cooking until wilted for another 2-3 minutes. Remove from heat and set aside.

In a medium saucepan, melt 4 tablespoons of butter over medium heat. Whisk in 4 tablespoons of all-purpose flour, cooking for 2-3 minutes until light golden. Gradually whisk in 3 cups of whole milk, stirring constantly until sauce thickens and bubbles, around 5-7 minutes. Season with salt, pepper, and a pinch of nutmeg.

Preheat your oven to 375°F (190°C). In a 9x13 inch baking dish, spread a thin layer of béchamel sauce on the bottom. Layer 3 no-boil lasagna noodles over the sauce, then add half of the mushroom and spinach mixture, followed by another layer of béchamel and a sprinkle of 1 cup of shredded mozzarella cheese. Repeat this process, finishing with a final layer of noodles, béchamel, and cheese on top.

Cover the lasagna with aluminum foil and bake for 30 minutes. Remove the foil and bake uncovered for an additional 15-20 minutes until the top is golden brown and bubbly.

Let the lasagna rest for about 10-15 minutes before serving.