In a large soup pot set over medium-high heat, melt the butter, then incorporate the chopped onion, minced garlic, and Italian sausage. Cook, breaking up the sausage with a spoon, until the meat is browned and the onion becomes soft and translucent. Stir in the chicken bone broth and fire-roasted tomatoes, followed by the cream cheese, ensuring it melts smoothly into the mixture. Incorporate the diced zucchini along with the dried basil, dried oregano, and black pepper; bring the soup to a boil before reducing the heat to maintain a gentle simmer for 15 to 20 minutes until the zucchini softens. Add the baby spinach and stir until it wilts, approximately 3 minutes. Ladle into bowls and garnish generously with shredded Parmesan cheese before serving.