Place chicken in bottom of slow cooker. Scatter chopped onion over chicken. Spoon cream of chicken soup over onions. Cover and cook on LOW 7-8 hours or on HIGH 3-4 hours.
Remove chicken and shred with two forks, return to slow cooker. Stir in cooked rice, corn and cheese, cover and cook until heated through, about 5-10 minutes more. Garnish with more grated cheddar cheese before serving, if desired.