Add in onion and sauté for 3-4 minutes until tender.
Add in garlic and cook for an additional minute.
Stir in chicken broth, diced tomatoes, crushed tomatoes or tomato sauce and Italian seasoning. Bring to a boil, then reduce to a simmer. Cook for 15 minutes.
Stir in tortellini and spinach and cook until warmed through.
Season with salt and pepper to taste.
Serve warm with basil and parmesan cheese for serving, if desired.
Notes
If you want to turn this into a creamy tomato tortellini soup with spinach, stir in 1/2 to 1 cup of whole milk or heavy whipping cream, at room temperature, after you have added the tortellini and spinach. (You don't want it to simmer with the other ingredients.) Adjust seasonings as needed.