In a large pot or Dutch oven over medium heat, cook the sausage, chopped onion, and Italian seasoning until the sausage is browned and the onion is softened, about 5–10 minutes.
Add crushed tomatoes, marinara sauce, and broth. Season with salt and pepper. Simmer, stirring occasionally, until slightly thickened, about 10 minutes.
Stir in 2 cups water and bring to a boil.
Add pasta and cook until tender, about 10 minutes, stirring occasionally to prevent sticking. Add spinach and stir until wilted.
Reduce heat and stir in heavy cream and mozzarella until melted and creamy.
Ladle into bowls and serve topped with Parmesan cheese and fresh herbs.