Step 1: Prepare the Vegetable Filling
Heat olive oil in a large skillet over medium heat.
Add mushrooms and sauté for about 5 minutes until softened and lightly browned.
Stir in spinach and garlic and cook until the spinach wilts.
Season with salt and black pepper, then remove from heat.
Step 2: Make the Béchamel Sauce
Melt butter in a saucepan over medium heat.
Whisk in flour and cook for 1 to 2 minutes to form a smooth paste.
Gradually whisk in warm milk, stirring constantly until smooth.
Simmer gently until the sauce thickens.
Season with nutmeg, salt, and black pepper. Remove from heat.
Step 3: Assemble the Lasagna
Preheat oven to 375°F (190°C).
Spread a thin layer of béchamel sauce in the bottom of a 9×13-inch baking dish.
Arrange 3 lasagna noodles over the sauce.
Top with one-third of the vegetable mixture, dollops of ricotta cheese, mozzarella cheese, and béchamel sauce.
Repeat the layers two more times.
Finish with the remaining béchamel sauce, mozzarella, and Parmesan cheese on top.
Step 4: Bake the Lasagna
Cover with aluminum foil and bake for 25 minutes.
Remove foil and bake an additional 15 minutes until golden and bubbly.
Let the lasagna rest for 10 minutes before slicing and serving.