1. Preheat your oven to 375°F (190°C).
2. Cook the lasagna noodles according to package instructions. Drain and set aside.
3. In a large skillet, heat olive oil over medium heat. Add the chopped onion and garlic, sautéing until the onion is translucent and fragrant.
4. Add the mushrooms to the skillet and cook until they are browned and have released their moisture, about 5-7 minutes.
5. Stir in the fresh spinach, salt, and pepper, cooking until the spinach is wilted. Set the mixture aside.
6. In a saucepan, melt the butter over medium heat. Stir in the flour and cook for 1-2 minutes, forming a roux.
7. Gradually whisk in the milk, continuing to stir until the sauce thickens, about 5 minutes. Stir in the nutmeg for a hint of warmth.
8. In a mixing bowl, combine the ricotta cheese with a portion of the mozzarella and Parmesan, reserving some for the topping.
9. To assemble, spread a thin layer of béchamel sauce on the bottom of a 9×13 inch baking dish. Layer three lasagna noodles, followed by half of the mushroom-spinach mixture and a portion of the ricotta mixture. Repeat the layers, finishing with noodles, béchamel, and the reserved mozzarella and Parmesan.
10. Bake in the preheated oven until the cheese is golden and bubbly, about 25-30 minutes. Let it cool slightly before serving.

Mama Lou always reminded us to let the lasagna rest before cutting into it, allowing the layers to set and making serving easier and more beautiful.