In a large skillet, heat 2 tablespoons of olive oil over medium heat. Sauté the sliced mushrooms for about 5 minutes until they turn golden brown. Next, add the roughly chopped spinach and minced garlic, cooking until the spinach wilts and reduces in size, about 2-3 minutes. Season with salt and pepper to taste, then set aside to cool while you make the béchamel sauce.

In a medium saucepan, melt 3 tablespoons of butter over low heat. Whisk in 3 tablespoons of all-purpose flour and cook for 1-2 minutes until bubbling and golden, creating a roux. Gradually add 3 cups of warm milk while whisking continuously to avoid lumps. Cook for 5-7 minutes, until the sauce thickens and coats the back of a spoon. Season with ½ teaspoon of ground nutmeg, salt, and pepper for flavor.

Preheat your oven to 375°F (190°C). Spread a thin layer of the prepared béchamel sauce at the bottom of a 9x13-inch baking dish to prevent sticking. Layer 3 lasagna noodles evenly over the sauce, followed by ⅓ of the sautéed vegetable filling, dollops of ricotta, a sprinkle of mozzarella, and a drizzle of béchamel. Repeat these layers twice more, ending with a final layer of béchamel topped with mozzarella and grated Parmesan cheese.

Cover the assembled lasagna with aluminum foil and place it in the preheated oven. Bake for 25 minutes to allow the layers to meld together. After this time, carefully remove the foil and bake for an additional 15 minutes. This step gives the cheese a chance to turn bubbly and golden brown, indicating it’s ready to be savored.

After removing the lasagna from the oven, allow it to rest for 10 minutes before slicing. This resting period helps stabilize the layers, making it easier to serve. Cut into squares and enjoy your deliciously comforting Creamy Spinach Mushroom Lasagna with your favorite side salad or garlic bread!