Remove tops, membranes, and seeds from sweet peppers. Chop enough tops to make 1/3 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 4-1/2-, 5- or 6-quart slow cooker; stir in reserved chopped pepper and onion. Place peppers, filled side up, in cooker.
Cover and cook on low-heat setting for 6 to 6-1/2 hours or on high-heat setting for 3 to 3-1/2 hours.
To serve, transfer peppers to serving plate and cut in half, if desired. Spoon tomato sauce over peppers and sprinkle with cheese and, if desired, chili powder.