Add onion, black beans, corn, tomato sauce, broth, diced tomatoes, and taco seasoning to a slow cooker.
Place the chicken breasts on top and push down until mostly submerged in the mixture.
Cook on low for 6-7 hours (or high for 2-3 hours, but the flavor won’t be as good).
Remove the chicken breasts from the soup and shred.
Add the chicken back to the pot.
To serve, top with optional toppings.
For the Instant Pot, combine all the ingredients for the soup in the pot and cook at high pressure for 10 minutes with a 10 minutes natural pressure release. Remove the chicken, shred, and return to the soup.Nutrition information does not include optional toppings.