Heat olive oil in a large stockpot or Dutch oven over medium heat. Add Italian sausage and cook until browned, about 3-5 minutes, making sure to crumble the sausage as it cooks.
Add onion and carrots, and cook, stirring occasionally, until tender, about 5 minutes.
Stir in garlic, Italian seasoning and fennel seeds until fragrant, about 1 minute.
Whisk in flour until lightly browned, about 1 minute.
Stir in chicken stock and Parmesan rind, if using; season with salt and pepper, to taste. Bring to boil; reduce heat and simmer, stirring occasionally, until reduced and slightly thickened, about 10 minutes.
Stir in tortellini and cook until tender, about 4-5 minutes.
Stir in spinach and heavy cream until the spinach has wilted, about 1-2 minutes. Stir in basil; season with salt and pepper, to toaste.