Heat the olive oil in a large pot over medium heat. Add the onion, garlic, carrots, and celery and sauté until the vegetables are tender, about 5 minutes.
Stir in the spinach, vegetable broth, water, tomatoes, Italian seasoning, salt, and black pepper. Bring to a boil, then reduce the heat to low and simmer for 10 minutes.
Stir in the tortellini and simmer for an additional 5-7 minutes, until the tortellini is cooked.
Serve the soup hot, garnished with Parmesan cheese if desired.