In a large soup pot, cook the Italian sausage over medium-high heat until fully cooked, about 8-10 minutes. Remove from the pot and drain off any excess fat.
Add the onion, garlic, and olive oil to the pot and cook until the onion is softened, about 5 minutes. Add the celery and carrot and cook for an additional 5 minutes.
Stir in the tomato paste, diced tomatoes, chicken broth, oregano, basil, red pepper flakes, salt, and black pepper and bring to a boil.
Reduce the heat to low and simmer for 30 minutes.
Add the cooked sausage and parsley to the pot and cook for an additional 5 minutes.
Serve hot.
Interesting Facts
Italian sausage is typically made with pork, but can also be made with beef, chicken, or turkey.
The red pepper flakes provide a subtle kick to the soup.
This soup is perfect served with a crusty bread or a side salad.