Heat olive oil in a large pot over medium heat. Add onion, carrots, and celery. Sauté for 5–7 minutes until vegetables begin to soften.
Add garlic and cook for 1 minute, stirring constantly, until fragrant.
Pour in chicken broth and water. Add chicken breast, thyme, oregano, salt, and pepper. Bring to a boil, then reduce heat to low, cover, and simmer for 15 minutes.
Remove chicken from the pot and shred it using two forks. Return shredded chicken to the pot.
Add tortellini to the soup and cook according to package instructions, usually 7–10 minutes, until tender.
Stir in chopped spinach and cook for 2–3 minutes until wilted.
Taste and adjust seasoning if needed. Serve hot, garnished with Parmesan cheese and fresh parsley if desired.
Notes
For a creamier soup, stir in 1/2 cup heavy cream or half-and-half with the spinach. Store leftovers in an airtight container in the refrigerator for up to 3 days.