In a large soup pot heat olive oil over medium heat and add onions, carrots, celery, garlic, and salt. Sauté veggies, stirring occasionally, for about 8-10 minutes – until soft and starting to caramelize.
Add tomato paste, poultry seasoning, oregano, salt, and chili flakes if using. Continue to cook until tomato paste has darkened slightly – another minute or two.
Add broth, diced tomatoes, and chicken breasts. Bring back up to a simmer. Simmer for about 20-25 minutes – until chicken is cooked through and veggies are tender. Reduce heat to low.