Prepare the vegetable filling. Heat the olive oil in a large skillet over medium heat. Add the sliced mushrooms and cook for about 5 minutes, stirring occasionally, until softened and lightly browned.

Add the chopped spinach and minced garlic. Cook until the spinach has wilted and excess moisture has mostly evaporated. Season with salt and black pepper, then remove from the heat.

Make the béchamel sauce. Melt the butter in a saucepan over medium heat. Add the flour and whisk continuously for 1–2 minutes to create a smooth paste.

Slowly pour in the warm milk while whisking constantly. Continue cooking over a gentle simmer until the sauce becomes smooth and thick enough to lightly coat the back of a spoon.

Season the béchamel with nutmeg, salt, and black pepper. Remove from the heat.

Preheat the oven to 375°F (190°C). Spread a thin layer of béchamel over the bottom of a 9×13-inch baking dish.

Arrange 3 lasagna noodles over the sauce. Add one-third of the spinach and mushroom mixture, followed by dollops of ricotta, some mozzarella, and a layer of béchamel.

Repeat the layers twice, finishing with béchamel sauce and the remaining mozzarella and Parmesan. This layering gives the Creamy Spinach Mushroom Lasagna its rich, tender texture.

Cover the baking dish with aluminum foil and bake for 25 minutes.

Remove the foil and continue baking for approximately 15 minutes, or until the cheese is melted, bubbling, and lightly golden.

Remove the lasagna from the oven and let it rest for 10 minutes before slicing and serving.