Slow Cooker Chicken Taco Soup

1 onion, chopped
16 oz chili beans, drained
15 oz black beans, drained
15 oz whole kernal corn, drained
8 oz can tomato sauce
12 oz chicken broth
2 10 oz cans diced tomatoes with green chillies, undrained
1.25 oz package taco seasoning
3 whole boneless chicken breasts

Place the onion, beans, corn, tomato sauce, chicken broth, and diced tomatoes in a slow cooker.  Add taco seasoning and stir to blend.  Lay chicken breasts on top of the mixture, pressing down slightly until just covered by the other ingredients.  Set slow cooker to low, cover and cook for 5 hours.

Remove chicken breasts from the soup and allow to cool long enough to be handled.  Shred the chicken (with 2 forks) then stir the shredded chicken back into the soup and continue cooking for 2 hours.  Serve topped with shredded cheese, sour cream, and crushed tortilla chips if desired.