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.