Heat 1 tablespoon of olive oil in a large soup pot over medium heat. Place the chicken sausage in the pot and cook until almost cooked through - breaking up the sausage as it cooks.
When the pink color in the sausage is just about gone, add the pepperoni and cook together until there is no pink left, about 3 minutes. Remove the sausage and pepperoni from the pan and place them into a bowl.
Add another tablespoon of olive oil to the same pot and saute the onion until translucent and fragrant, about 3 minutes. Stir in the garlic and cook for another 2-3 minutes or until fragrant.
Stir in the mushrooms and cook until soft, about 5 minutes.
Stir the Italian seasoning, sausage, and pepperoni into the mushrooms, onion, and garlic.
Pour in the chicken broth, diced tomatoes, tomato sauce, and paste.
Bring the pizza soup to a boil, reduce it to a simmer, and cook for 45 minutes.
Give it a taste and add salt and pepper, if needed.
Ladle into individual bowls and serve with shredded mozzarella.