In a stock pot, cook onion & garlic in olive oil till onion is translucent (medium heat). Do not burn garlic. Add mushrooms if using & cook till tender.
Increase heat to HI and add broth and diced tomatoes with juice. Cover and bring to a boil then reduce heat. simmer 10 minutes.
Dice chicken discarding skin and bones. Add frozen spinach to soup, raise heat to keep at simmer if needed. Cook 5 minutes. Add tortellini, cook 5 minutes. Add chicken, simmer till hot. Top each serving with grated parmesan if desired.