In a large pot, heat olive oil over medium heat.
Add diced onions, carrots, and celery; sauté until softened.
Stir in minced garlic and cook for an additional minute.
Season chicken breasts with salt and pepper, then add to the pot.
Pour in chicken broth and bring to a boil.
Reduce heat and simmer for 15 minutes or until chicken is cooked through.
Remove chicken, shred it, and set aside.
Add orzo to the pot and cook according to package instructions.
Stir in lemon juice and zest, then return shredded chicken to the soup.
Adjust seasoning, garnish with fresh parsley, and serve.