Preheat the broiler in your oven.
Sauté the onions and garlic in a large saucepan or soup pot until the onions are lightly browned and your kitchen smells amazing. Add the tomatoes, crushing them with the back of a wooden spoon. Add the chicken broth, oregano and carrots. Add the pepperoni and season with salt and pepper to taste. Simmer for 15 minutes.
Ladle the hot soup into 4 onion soup bowls or four large soup cups. Cut the bread into large circles that fit inside the soup bowls and place one onto each bowl of soup. Add half a cup of grated mozzarella to the top of each bowl and place under broiler. Cook until the cheese is golden and bubbling, about 1-2 minutes.