Brown sausage and onion in a 6-quart pot over medium-high heat for 7 minutes, breaking sausage into small crumbles and cooking it to 160°F. Add pepperoni for 1 minute and drain excess fat.
Stir in green pepper and mushrooms and cook for 5 minutes until mushrooms soften. Add garlic and oregano for 30 seconds.
Pour in tomatoes and broth, then add black pepper. Bring to a simmer, scraping the bottom.
Simmer uncovered for 10 to 12 minutes until peppers are tender and the soup is hot throughout.
Divide among 6 bowls and add the mozzarella evenly over the hot soup. Let it soften for 1 minute before eating.