Heat the olive oil in a large soup pot over medium heat, add the chicken breast, and lightly brown it for about 2 minutes on each side.
Add the onion, carrots, and celery around the chicken, then cook for 4 to 5 minutes until the vegetables begin to soften.
Pour in the chicken broth and diced tomatoes, then stir in the thyme, garlic powder, salt, and black pepper.
Bring the soup to a boil, reduce the heat, cover, and simmer for about 15 minutes or until the chicken is fully cooked.
Remove the chicken, shred it with two forks, and add the green beans and zucchini to the simmering broth.
Cook the vegetables for another 8 to 10 minutes, then return the shredded chicken to the pot.
Stir in the lemon juice and parsley, taste the broth, and serve once everything is heated through.