In a large skillet, heat 2 tablespoons of olive oil over medium heat. Sauté 8 ounces of sliced mushrooms for about 5 minutes, stirring occasionally, until golden brown. Add 4 cups of chopped spinach and 2 minced garlic cloves, seasoning with salt and pepper to taste. Cook until the spinach wilts down, which takes about 2-3 minutes, then set aside to cool slightly.

In a saucepan, melt 4 tablespoons of butter over medium-low heat. Whisk in 1/4 cup of all-purpose flour, stirring constantly for 1-2 minutes until bubbly but not browned. Gradually pour in 3 cups of warm milk while continuing to whisk, cooking until the mixture thickens and bubbles, about 5-7 minutes. Season with a pinch of nutmeg, salt, and pepper for added flavor.

If using no-boil lasagna noodles, skip this step. Otherwise, bring a large pot of salted water to a boil and cook the noodles according to package instructions until just tender, about 8-10 minutes. Drain and lay them flat on a clean kitchen towel to prevent sticking while you assemble the Easy Creamy Spinach Mushroom Lasagna.

Preheat your oven to 375°F (190°C). In a baking dish, spread a thin layer of béchamel sauce to coat the bottom. Add a layer of lasagna noodles, followed by half of the vegetable filling, dollops of 1 cup of ricotta cheese, and a sprinkle of 1 cup shredded mozzarella. Pour more béchamel sauce over the top. Repeat this layering process with remaining ingredients, finishing with noodles and béchamel.

Cover the assembled Easy Creamy Spinach Mushroom Lasagna with aluminum foil to retain moisture. Bake in the preheated oven for 25 minutes. Remove the foil, sprinkle with remaining mozzarella and ½ cup grated Parmesan cheese, then bake uncovered for an additional 15 minutes until the top is golden and bubbly.

After baking, let the lasagna rest for 10 minutes to firm up, making slicing easier. Cut into squares and serve warm, enjoying the creamy layers of veggies and pasta.