01 -  Preheat oven to 375°F (190°F). 
 02 -  In a large skillet over medium heat, cook the ground beef with diced onion and minced garlic, stirring, until the meat is browned and the onion is soft. 
 03 -  Transfer the cooked beef mixture to a bowl. Add cooked rice, tomato sauce, Italian seasoning, salt, and pepper. Mix until thoroughly combined. 
 04 -  Arrange the prepared bell peppers upright in a baking dish. Fill each pepper evenly with the beef and rice mixture. 
 05 -  Sprinkle shredded mozzarella cheese on top of the filled peppers. 
 06 -  Cover the baking dish with aluminum foil and bake for 35 minutes. Remove foil and bake for an additional 10 minutes, until peppers are tender and cheese is golden.