Slow Cooker Chicken Taco Stew

1 onion, chopped
1 16-oz can black beans
1 16-oz can kidney beans
1 16-oz can corn (drained)
1 8-oz can tomato sauce
2 14.5-oz cans diced tomatoes w/chilies
1 1.25-oz packet taco seasoning
1-2 boneless skinless chicken breasts

Mix everything together in a slow cooker except chicken. Lay chicken on top and cover. Cook on low for 6-8 hours or on high for 3-4 hours. 30 minutes before serving, remove chicken and shred. Return chicken to slow cooker and stir in. This is good eaten with cheese, sour cream, or tortilla chips.