1In a large pot, combine the chicken broth, marinara sauce, and diced tomatoes. Bring to a simmer over medium heat.
Add the sliced pepperoni, diced green bell pepper, diced onion, and sliced mushrooms to the pot. Stir well.
Season the soup with Italian seasoning and red pepper flakes. Stir to combine.
Let the soup simmer for about 15-20 minutes, or until the vegetables are tender.
Ladle the soup into bowls and top each serving with shredded mozzarella cheese.
Serve hot and enjoy your Classic Pizza Soup!