In a large skillet over medium heat, brown the ground beef with the chopped onion, salt, pepper, garlic powder and Italian seasoning until the beef is fully cooked and the onion is tender.
Drain excess grease, then transfer the mixture to the slow cooker.
Add the chopped red and green bell peppers, diced tomatoes (with juices), tomato sauce, Worcestershire sauce, and beef broth to the slow cooker and stir to combine.
Cover and cook on low for 6 to 7 hours or on high for 3 to 4 hours, until the peppers are tender and flavors are well blended.
About 20 minutes before serving, cook the white rice separately according to package directions.
Just before serving, stir the cooked rice into the soup and let it sit for 5 minutes to thicken slightly.
Ladle into bowls and top each serving with shredded mozzarella cheese, if desired.