Heat a tablespoon of olive oil in a large skillet over medium heat. Sauté sliced mushrooms until golden brown, about 5 minutes. Add chopped fresh spinach and minced garlic, cooking until the spinach is wilted, approximately 2 minutes. Season with salt and pepper, then transfer to a bowl to cool.

In a medium saucepan, melt 4 tablespoons of butter over medium heat. Whisk in 1/4 cup of all-purpose flour, stirring constantly to form a roux for about 2 minutes. Gradually pour in 2 cups of warm milk while whisking. Cook until it thickens and bubbles, around 5 minutes. Season with nutmeg, salt, and pepper.

If you’re not using no-boil noodles, cook your lasagna sheets in a large pot of boiling salted water according to package instructions. Drain and lay them flat on a clean kitchen towel.

Preheat your oven to 375°F (190°C). Spread a thin layer of béchamel sauce on the bottom of a 9x13-inch baking dish. Layer lasagna noodles, then add 1/3 of the vegetable filling, dollops of ricotta, and a sprinkle of mozzarella. Repeat two more times, finishing with noodles on top.

Pour any remaining béchamel sauce over the top layer and scatter remaining mozzarella and grated Parmesan. Cover with aluminum foil and bake for 25 minutes. Remove foil and bake for an additional 15 minutes until golden brown and bubbling.

Let the lasagna rest for 10 minutes before slicing to set the layers.