Place the onion, chili beans, black beans, corn, tomato sauce 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 for low heat, cover, and cook for 3 to 4 hours (4 to 5 if yoru chicken is frozen), or until chicken is done and shreds easily. (*See note)
Remove chicken breasts from the soup and shred. Add shredded chicken back into the slow cooker and give it a stir.
Serve topped with desired toppings.
I recommend cooking this soup on low heat. Chicken breasts are a lean cut of meat, which means meaning they are low in fat. Because fat helps to keep meat tender and juicy as it cooks, chicken breasts are easy to overcook, which results in tougher, drier chicken.
This is a thicker soup, so if you want it on the “soupier” side then go ahead and add some chicken broth or water. We love it just as I’ve written the recipe with no liquid added.