1. Preheat your oven to 350°F (175°C). Cook the lasagna noodles according to the package instructions, then drain and set aside.
2. In a large skillet, heat the olive oil over medium heat. Add the chopped onion and sauté until translucent, about 5 minutes.
3. Add the garlic and sliced mushrooms to the skillet. Cook, stirring occasionally, until the mushrooms are browned and softened, approximately 7-8 minutes.
4. Stir in the fresh spinach and cook until wilted, about 3 minutes. Season with salt, pepper, and oregano. Remove from heat and set aside.
5. In a saucepan, melt the butter over medium heat. Whisk in the flour and cook for 1 minute.
6. Slowly whisk in the milk, stirring constantly until the mixture is smooth and thickens, about 5 minutes. Add the nutmeg, and season with salt and pepper.
7. In a large bowl, combine the ricotta cheese, half of the Parmesan cheese, and 1 cup of mozzarella cheese. Mix well.
8. To assemble the lasagna, spread a small amount of the cream sauce on the bottom of a 9×13 inch baking dish. Place three lasagna noodles on top.
9. Spread half of the ricotta mixture over the noodles, followed by half of the spinach and mushroom mixture, and then a third of the remaining cream sauce.
10. Repeat the layers, ending with the last three lasagna noodles, the remaining cream sauce, and the rest of the mozzarella and Parmesan cheese.
11. Cover with foil and bake in the preheated oven for 25 minutes. Remove the foil and bake for an additional 15 minutes, or until the top is golden and bubbly.
12. Let it rest for 10 minutes before slicing and serving.

This dish is a labor of love, much like the meals I helped my abuela prepare. Remember, the key is in the layering and embracing the aromas that fill your kitchen as it bakes.