Brown the sausage: Heat a large Dutch oven or soup pot over medium heat. Add the Italian sausage and cook for 6–8 minutes, breaking it into small pieces as it browns. Drain excess grease if necessary.
Cook the aromatics: Add the diced onion and cook for 4–5 minutes, until softened. Add the minced garlic and cook for another 30–60 seconds.
Add the broth and seasonings: Pour in the chicken broth. Stir in the Italian seasoning, red pepper flakes, black pepper, and salt.
Simmer: Bring the soup to a gentle simmer. Cook for about 10 minutes to allow the flavors to combine.
Add the spinach: Stir in the baby spinach and cook for 2–3 minutes, until wilted.
Add the cream: Reduce the heat to low and slowly stir in the heavy cream. Do not bring the soup to a vigorous boil.
Add the Parmesan: Gradually stir in the freshly grated Parmesan until completely melted and incorporated into the broth.
Adjust the seasoning: Taste the soup and add additional salt, pepper, or red pepper flakes as desired. If the soup is too thick, add a little extra chicken broth.
Serve: Ladle the hot soup into bowls and garnish with additional Parmesan and fresh parsley, if desired. Serve immediately with crusty bread.
