Add mushrooms (optional)and cook until tender, stirring occasionally. Increase heat to high and pour in broth and diced tomatoes w liquid. Cover and bring to a boil, then reduce heat and simmer for 10 minutes. Dice chicken and set aside. Add frozen spinach to soup, raising heat to keep at a simmer. Cook 5 more minutes and then stir in diced chicken. Serve immediately, topping each with freshly grated parmesan.
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