Add the ground beef or Italian sausage to the Dutch oven or large pot with the onions, bell peppers, garlic, Italian Seasoning, salt, and pepper.
Add the pepperoni, marinara sauce, fire-roasted tomatoes, and broth to the pot and stir.
Adjust the heat to low and place a lid on the pot. Simmer for 15-20 minutes until thick and the ingredients are well combined.
Top the soup with shredded mozzarella cheese.