Preheat the oven to 375°F (190°C). Grease a 9x13-inch baking dish with oil or butter.

Heat olive oil in a large skillet over medium heat. Add minced garlic and sliced mushrooms and sauté for 5-7 minutes, until browned and tender.

Stir in the spinach and cook until wilted, or just heated through if using frozen. Season with salt and pepper and set the mixture aside.

Melt butter in a saucepan over medium heat. Whisk in flour and cook for 1-2 minutes until light golden, watching closely so it doesn't burn.

Gradually whisk in the milk until smooth. Bring to a simmer and cook 5-7 minutes until thickened, then season with salt, pepper, and nutmeg if using.

Spread a thin layer of béchamel on the bottom of the baking dish and lay down a layer of lasagna noodles.

Top with some of the spinach-mushroom mixture, dollops of ricotta, and a sprinkle of mozzarella and Parmesan. Repeat the layering until you run out, ending with noodles, béchamel, and the remaining mozzarella and Parmesan.

Cover with foil and bake for 25 minutes. Remove the foil and bake uncovered another 10-15 minutes, until the top is golden and bubbly.

Let the lasagna rest for 10 minutes before slicing. Garnish with fresh parsley if desired and serve warm.