Trim tops off bell peppers; remove ribs and seeds. Set aside.
Place beef in large bowl; set aside.
In 6-inch skillet, heat olive oil over medium-high heat; add chopped onion. Cook until onion softens, stirring every few minutes. Add garlic; cook 60 seconds. Remove from the heat; cool slightly.
Add rice, salt and pepper to beef. Add cooked onion and garlic; mix to combine. Stuff peppers with beef mixture; arrange in slow cooker. Pour tomato sauce over peppers.
Cook on Low heat setting 6 hours; last 30 minutes of cooking, top peppers with cheese.