Heat a heavy pot over medium heat. Add the sausage and cook until browned, breaking the sausage apart as it cooks. This takes 10-12 minutes.
Transfer the sausage to a paper towel-lined plate. Drain excess fat, leaving about 2 teaspoons in the pot.
Add one tablespoon of butter to the drippings in the pot. Add the onion and a pinch of salt and pepper and cook until the onions are softened, 2-3 minutes. Add the garlic and cook for one minute. Stir in the tomato paste and cook for another minute.
Add the chopped tomatoes and chicken stock. Bring to a gentle boil, then turn down and simmer for 10 minutes. (Note 1)
Add the tortellini and sausage and simmer for another 5 minutes. (Note 2) Stir in heavy cream and remaining tablespoon of butter. Taste for seasoning and add another pinch of salt and pepper if needed.
Ladle into bowls and garnish with fresh basil.
Notes
Optional: At this point, if you prefer a smooth soup, you can blend the soup with an immersion blender before adding the sausage and pasta.
If using fresh tortellini, reduce simmer time by 1-2 minutes.