Begin by heating olive oil in a large skillet over medium heat. Add the chopped onion and sauté until translucent, about 5 minutes. Then stir in the sliced mushrooms and cook until they release their moisture and become golden brown, around 10–12 minutes. Add minced garlic, salt, and pepper and cook for an additional minute until fragrant. Once the mushrooms are cooked, add the fresh spinach in batches, stirring until wilted. Set the vegetable mixture aside.

Next, prepare the béchamel sauce by melting butter in a saucepan over medium heat. Whisk in the flour to form a smooth roux, cooking for 1–2 minutes until it slightly darkens. Slowly pour in the milk while whisking constantly to prevent lumps. Cook the sauce until it thickens and coats the back of a spoon, about 6–8 minutes. Stir in the nutmeg, then remove from heat.

Now it’s time to assemble the lasagna. Preheat your oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish. Spread a small amount of béchamel sauce on the bottom of the dish, followed by a layer of noodles. Add a layer of the spinach and mushroom mixture, a few dollops of ricotta, and a generous sprinkle of mozzarella. Repeat the layers—béchamel, noodles, vegetables, ricotta, and cheese—until all ingredients are used up, finishing with a layer of béchamel and a topping of mozzarella and Parmesan cheese.

Cover the lasagna with foil and bake for 25–30 minutes. Remove the foil and bake for an additional 10–15 minutes, or until the top is golden and bubbling. Let it rest for at least 10 minutes before slicing and serving. This allows the layers to set beautifully.