In a large saucepan over medium heat, add the Italian sausage, breaking it apart with a spatula. Cook for about 5-7 minutes, or until the sausage is browned and cooked through, then add the diced onion. Sauté for another 3-4 minutes until the onion is translucent, stirring occasionally. Drain any excess grease before proceeding.

Stir in the minced garlic, optional ground fennel, and red pepper flakes to the sausage mixture in the saucepan. Cook for approximately 1 minute, stirring frequently, until the garlic becomes fragrant and slightly golden.

Pour in the chicken broth and diced tomatoes, then add the pasta and Italian seasoning. Increase the heat to high, bringing the mixture to a gentle boil. Once boiling, reduce the heat to low and let simmer for about 6-8 minutes until the pasta is al dente.

Gently fold in the grated Parmesan cheese, cream cheese, and heavy cream into the simmering soup. Stir continuously for approximately 3-5 minutes, allowing the cheeses to melt fully.

Taste the soup and adjust the seasoning with salt and pepper as desired. Just before serving, sprinkle in fresh parsley.