Heat 1 tablespoon of olive oil in a large skillet over medium heat. Add the mushrooms, season with salt and pepper, and sauté until tender and golden brown, about 5-7 minutes. Remove and set aside.

In the same skillet, add the remaining olive oil and sauté the spinach until wilted, about 3-4 minutes. Season with a pinch of salt and pepper. Set aside.

In a medium saucepan, melt the butter over medium heat.

Stir in the flour and cook for 1-2 minutes to form a roux.

Slowly pour in the milk, whisking constantly to avoid lumps. Bring to a gentle simmer, stirring until the sauce thickens.

Add garlic powder, onion powder, nutmeg (if using), salt, and pepper. Remove from heat and set aside.

In a medium bowl, mix the ricotta cheese, egg, and half of the Parmesan cheese. Season lightly with salt and pepper.

Preheat the oven to 375°F (190°C). Spread a thin layer of béchamel sauce at the bottom of a 9×13-inch baking dish. Add a layer of lasagna noodles, followed by ricotta mixture, sautéed spinach, mushrooms, béchamel sauce, and a sprinkle of mozzarella cheese. Repeat the layers, ending with a layer of béchamel sauce and mozzarella on top.

Finish with the remaining Parmesan cheese for a golden crust.

Cover the dish with foil and bake for 30 minutes.

Uncover and bake for an additional 15-20 minutes, or until the top is bubbly and golden brown.

Let the lasagna rest for 10-15 minutes before slicing and serving.