In a medium dutch oven or soup pot over medium-high heat, saute chicken, carrots, celery, onion, and garlic until veggies are tender and chicken is no longer pink.
Reduce heat to medium and add chicken broth. Bring to a low simmer, stirring occasionally, then add cooked orzo, baby spinach, lemon juice, zest, oregano, and salt and pepper to taste.
Bring soup to a low simmer until heated through. Ladle soup into bowls and top with crumbled feta. Enjoy!