Remove tops, membranes and seeds from peppers. Chop enough tops to make ⅓ cup; set aside. If necessary, cut a thin slice from the bottom of each pepper so they sit flat. In a medium bowl, stir together rice and undrained beans; spoon into peppers. Pour tomato sauce into the bottom of a 4½-, 5- or 6-quart slow cooker; stir in reserved chopped pepper and onion (see Tip). Place peppers, filled-side up, in cooker.
Cover and cook on Low for 6 to 6½ hours or on High for 3 to 3½ hours.
To serve, transfer peppers to serving plate and cut in half, if desired. Spoon tomato sauce over peppers and sprinkle with cheese.
Tip: for easy cleanup: Line your slow cooker with a disposable slow cooker liner. Add ingredients as directed in recipe. Once your dish is finished cooking, spoon the food out of your slow cooker and simply dispose of the liner. Do not lift or transport the disposable liner with food inside.