Heat 1 tablespoon of olive oil in a large pot or Dutch oven over medium-high heat. Add 1 pound of Italian sausage, breaking it up with a wooden spoon, and cook for 5-7 minutes until it is browned and no longer pink.
Add the finely chopped onion to the pot and cook for 4-5 minutes, stirring occasionally, until the onion becomes soft and translucent.
Stir in the minced garlic and 2 teaspoons of Italian seasoning and cook for 1 minute until the garlic becomes fragrant.
Pour in 6 cups of chicken broth and the entire can of undrained diced tomatoes, scraping the bottom of the pot to loosen any browned bits. Bring the soup to a boil.
Add 1 cup of dried elbow macaroni to the boiling soup, reduce the heat to a simmer, and cook for 10-12 minutes, stirring occasionally, until the pasta is tender.
Turn off the heat and stir in 4 cups of fresh spinach and 1/4 teaspoon of black pepper. The spinach will wilt into the hot soup within 1-2 minutes. Ladle the soup into bowls and top each serving with a sprinkle of Parmesan cheese.
