Preheat the oven to 375°F (190°C). Cook the lasagna noodles according to package instructions until al dente. Drain and set aside.

In a large skillet, heat olive oil over medium heat. Add the diced onion and sauté for about 3 minutes until translucent. Stir in the minced garlic and cook for an additional minute.

Add the sliced mushrooms to the skillet and cook until they are browned and have released their moisture, about 5-7 minutes. Stir in the fresh spinach, oregano, salt, and pepper. Cook until the spinach is wilted, about 2-3 minutes. Remove from heat and let cool slightly.

In a large bowl, combine the ricotta cheese, half of the mozzarella cheese, and half of the Parmesan cheese. Mix in the cooled mushroom and spinach mixture until well combined.

Spread a thin layer of marinara sauce on the bottom of a 9x13-inch baking dish. Take one lasagna noodle and place about 1/4 cup of the filling mixture on one end. Roll the noodle tightly and place it seam-side down in the baking dish. Repeat with the remaining noodles and filling.

Once all the rolls are in the dish, pour the remaining marinara sauce over the top. Sprinkle the remaining mozzarella and Parmesan cheese on top.

Cover the dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 15 minutes, or until the cheese is bubbly and golden.

Let the lasagna rolls cool for about 5 minutes before serving. Garnish with fresh basil if desired.