Warm the olive oil in a large pot over medium heat.
Sauté onion, carrots, and celery until slightly soft, about 5–6 minutes.
Stir in garlic, Italian seasoning, red pepper flakes, salt, and black pepper. Cook 30 seconds.
Pour in chicken broth and bring to a gentle simmer.
Add shredded chicken and simmer 5 minutes to blend flavors.
Increase heat to a lively simmer, then add tortellini. Cook until tender, 4–6 minutes.
Stir in spinach to wilt, then add parsley and lemon juice.
Taste and adjust seasoning. Ladle into bowls and top with Parmesan.