In a large pot, heat a tablespoon of oil over medium heat.
Add the diced onion, carrots, and celery; sauté until soft.
Stir in the minced garlic and cook for another minute.
Add the chicken breasts and pour in the chicken broth.
Bring to a boil, then reduce to a simmer.
Cook the chicken for about 15 minutes, until fully cooked.
Remove the chicken, shred it, and set aside.
Add the orzo to the pot and cook according to package instructions.
Stir in the shredded chicken, lemon juice, zest, and spinach.
Season with salt and pepper to taste, then serve hot.