Warm olive oil in a large, deep skillet over medium heat. Sauté onion, mushrooms,
and green bell peppers until soft, but not browned. It usually takes about 3 to
5 minutes for the onions to become translucent.
Add stewed tomatoes, beef stock, pepperoni, basil, oregano and bay leaf. Continue
to warm until the mixture is heated through, making sure to stir occasionally. Discard
the bay leaf. Add salt and pepper to taste.
Ladle the soup into ovenproof bowls and sprinkle with shredded mozzarella cheese.