01 - Preheat oven to 375°F (190°C).
02 - In a large skillet over medium heat, cook ground beef and diced onion until beef is browned and onion is tender. Drain excess fat if needed.
03 - Stir in the drained diced tomatoes, cooked rice, Italian seasoning, salt, and black pepper. Mix until thoroughly combined and heated through.
04 - Slice the tops off the bell peppers and remove seeds and membranes. Fill each pepper with the beef and rice mixture, gently pressing to pack.
05 - Sprinkle shredded mozzarella evenly over each stuffed pepper.
06 - Arrange the stuffed peppers in a baking dish. Cover with aluminum foil and bake for 30 to 35 minutes, or until peppers are fork-tender and cheese is melted.