In a 5 1/2 quart stockpot, heat olive oil over medium heat. Add in the chopped onion and saute until translucent, about 5 minutes.
Stir in the garlic and cook t until fragrant, about 3 minutes.
Add in the carrots and celery then cook for about 4-5 minutes.
Add the chicken breast, chicken stock, diced tomatoes, and tomato sauce.
Bring to a boil, then reduce to a simmer and cook for 30 minutes.
Carefully remove the chicken and cut it into 1-inch chunks. Add the chicken back into the soup and cook for another 1/2 hour.