Add onion and cook until softened, 4 to 5 minutes.
Stir in garlic and cook until fragrant, about 30 seconds.
Transfer onion mixture to a 6-quart slow cooker. Add tomatoes, carrots, tomato paste, Italian seasoning, salt, pepper, and broth. Stir to combine.
Cover and cook on low for 4 to 6 hours or high for 3 to 4 hours, until carrots are tender.
About 20 minutes before serving, set the slow cooker to high if using low. Stir in tortellini and cook until just tender, 10 to 15 minutes (see Notes).
Stir in spinach and cook until wilted, about 5 minutes.
Gradually stir in cream and Parmesan until melted and smooth.
Notes
This recipe uses dried tortellini. If using fresh or frozen tortellini, reduce cooking time in Step 6 to 5 to 7 minutes or until pasta is just tender.
Stir in a tablespoon of butter or finish with a swirl of pesto.
Refrigerate for up to 3 days. Reheat gently; add a splash of broth to loosen if needed.