Beans, tomato, onion: it's a hearty vegetarian chili stuffed inside a bell pepper. If you're looking for extra kick, try topping with pepper jack instead of mozzarella.
Remove tops, seeds, and membranes 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 it will stand flat.
In a medium bowl stir together beans and cooked rice; spoon into peppers. Pour tomato sauce into a 4 1/2- to 6-quart slow cooker; stir in onion and the reserved 1/3 cup chopped pepper. Place filled peppers, filling sides 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 dinner plates and, if desired, cut in half. Spoon tomato mixture over peppers. Sprinkle with cheese and, if desired, chili powder.