Bring a large pot of salted water to boil. Add the 9 lasagna noodles and cook until al dente, usually 8 to 10 minutes. Drain carefully and lay them flat on a clean kitchen towel or parchment paper to prevent sticking.
Heat 2 tablespoons of olive oil in a large skillet over medium heat. Add 1 cup diced onion and cook until softened, about 3 minutes. Stir in 3 cloves minced garlic and cook for another minute until fragrant. Add 8 ounces sliced mushrooms and sauté until golden brown, around 5 to 7 minutes. Stir in 5 cups chopped fresh spinach and cook just until wilted. Season with 1 teaspoon Italian seasoning, salt, and pepper to taste. Remove from heat and let cool slightly.
In a medium bowl, combine 1 cup ricotta cheese, 1 beaten egg, and 1 cup grated Parmesan cheese. Fold in the cooked vegetable mixture until evenly blended.
Preheat oven to 375°F (190°C). Spread about 1/2 cup marinara sauce across the bottom of the baking dish. Lay one lasagna noodle on a flat surface, spread about 1/6 of the cheese and vegetable filling evenly along the length of the noodle. Carefully roll up the noodle and place seam-side down in the baking dish. Repeat with remaining noodles and filling.
Pour remaining 1 1/2 cups marinara sauce evenly over the rolled noodles. Sprinkle 1 cup shredded mozzarella cheese on top. Cover dish with foil and bake for 25 minutes. Remove foil and bake another 10 minutes or until cheese is bubbly and golden.
Let the Spinach Mushroom Lasagna Roll Bake rest for 5 minutes before serving to allow rolls to set. Serve with a crisp green salad or garlic bread if desired.