Preheat your oven to 375°F (190°C).
Cook the lasagna noodles in a large pot of boiling salted water according to package instructions. Drain and set aside.
In a sauté pan, heat the olive oil over medium heat. Add the chopped onion and minced garlic, sautéing until the onion becomes translucent.
Add the sliced mushrooms to the pan and cook until they are softened, about 5-7 minutes. Stir in the fresh spinach and cook until wilted. Season with salt, black pepper, and dried oregano. Remove from heat.
In a mixing bowl, combine ricotta cheese, Parmesan cheese, heavy cream, salt, and black pepper. Mix until well combined.
To assemble the lasagna, spread a thin layer of the ricotta mixture on the bottom of a baking dish. Layer 3 lasagna noodles on top, followed by half of the spinach and mushroom mixture, and half of the remaining ricotta mixture. Repeat the layers, finishing with the last 3 noodles.
Pour any remaining ricotta mixture on top and sprinkle with shredded mozzarella cheese.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes. Then, remove the foil and bake for an additional 15 minutes or until the cheese is bubbly and golden.
Allow the lasagna to cool for about 10 minutes before slicing and serving.