Cook the noodles: Boil lasagna noodles according to package directions. Drain and set aside. If using no-boil noodles, skip this step.

Sauté the vegetables: In a large skillet, heat olive oil over medium heat. Add the diced onion and cook until soft, about 5 minutes. Add garlic and cook for 1 minute more.

Add mushrooms: Stir in the sliced mushrooms and cook until they’re golden and all moisture has evaporated, about 7–10 minutes.

Add spinach: Stir in fresh spinach and cook until wilted. If using frozen spinach, make sure it’s thawed and well-drained. Season with nutmeg, salt, and pepper. Remove from heat.

Make the béchamel sauce: In a saucepan, melt butter over medium heat. Whisk in the flour and cook for 1 minute. Gradually whisk in the milk and cook, stirring constantly, until the sauce thickens (about 5–7 minutes). Stir in Parmesan cheese and a pinch of salt.

Assemble the lasagna: Preheat oven to 375°F (190°C). Spread a thin layer of béchamel sauce on the bottom of a 9×13-inch baking dish. Layer with 3 noodles, half of the mushroom-spinach mix, spoonfuls of ricotta (if using), a handful of mozzarella, and a layer of béchamel. Repeat the layers. Finish with noodles, the rest of the béchamel, and top with remaining mozzarella.

Bake: Cover with foil and bake for 25 minutes. Uncover and bake an additional 15–20 minutes until bubbly and golden on top.

Rest and serve: Let the lasagna rest for 10 minutes before slicing. Garnish with fresh parsley.