In a large pot, cook the Italian sausage over medium heat until browned, about 5 minutes. Remove from the pot and set aside.
Heat the olive oil in the same pot over medium heat. Add the onion, garlic, carrots, and celery and cook until the vegetables are softened, about 5 minutes.
Add the chicken broth, tomatoes, Italian seasoning, red pepper flakes, salt and black pepper and bring to a boil. Reduce heat to low and simmer for 15 minutes.
Return the cooked sausage to the pot and simmer for an additional 15 minutes.
Stir in the parsley and serve.
Interesting Facts
The Italian sausage used in this soup can be either sweet or spicy, depending on your preference.
This soup can be served with crusty bread or a side salad for a complete meal.
This soup can be made in advance and reheated when ready to serve.