Brown ground beef in a skillet over medium-high heat with onion and garlic, breaking it up, until no pink remains, about 5 minutes. Drain excess fat. Stir in 1 cup tomato sauce, drained diced tomatoes, Worcestershire sauce, Italian seasoning, salt, and pepper. Remove from heat and mix in the cooked rice.

Cut the tops off bell peppers and discard seeds and membranes. Trim a tiny slice off the bottom if needed to help them stand upright, but don’t cut through.

Spoon the beef mixture into each pepper, packing it gently. Pour the remaining tomato sauce into the bottom of a 6-quart slow cooker. Place stuffed peppers upright in the sauce.

Cover and cook on low for 4 hours, until peppers are tender when pierced with a fork. Uncover, top each pepper with shredded cheese, then cover again and cook on low 15 more minutes until cheese melts. Serve hot.