In a large, heavy bottom pot (I use an enameled cast iron pot), heat the avocado oil over medium heat. Add onions and carrots and season with salt and pepper. Cook, stirring occasionally, for about 5 minutes.
Add your cooked and chopped chicken sausage, letting brown slightly for an additional 3-4 minutes.
Add the garlic, fennel, Italian seasoning, and red pepper flakes, mix, and cook until fragrant, about a minute.
Add the broth, Parmesan rind (if using), and tomatoes, bring to a boil, reduce the heat and simmer with the lid on top for about 10 minutes.
Remove lid and remove Parmesan rind. Add pasta, cooking for about 6-7 minutes.
Mix in spinach and heavy cream. Let cook for an additional 2-3 minutes and then turn off heat. Taste and season with additional salt and pepper if needed.
Serve with grated Parmesan on top if desired!
I used a fully cooked and casings removed chicken sausage which I just chopped into some pieces. If you find an uncooked chicken sausage that you would rather use, I would recommend cooking it at the same time as the onion and carrots, breaking and crumbling the sausage apart as it cooks.