Preheat the oven to 375°F.
In a large pot of boiling salted water, cook the pasta according to package instructions; drain well and set aside.
In a large skillet over medium heat, melt the butter.
Add the onion and mushrooms and cook for 2-3 minutes, or until the mushrooms are soft and tender.
Stir in the garlic, spinach, Italian seasoning, salt, and pepper. Cook for another minute.
Whisk in the flour and cook until lightly browned, about 1 minute.
Gradually whisk in the vegetable broth followed by the cream, cooking and whisking constantly until the mixture is smooth and combined, about 1-2 minutes.
Stir in the Parmesan just before removing the skillet from the heat.
Place the cooked pasta in a large 13×9-inch baking dish.
Pour the spinach and mushroom cream sauce over the pasta and sprinkle the mozzarella cheese evenly on top.
Bake for 18-20 minutes, or until bubbly.