Add chopped onion to the bottom of your slow cooker.
Place chicken on top of the onion. Don't worry about defrosting if your chicken breasts are frozen.
In a small bowl combine the salt, pepper, garlic powder, onion powder, cumin, and chili powder. Sprinkle seasoning over the chicken.
Pour chicken broth into slow cooker over the chicken.
If using frozen chicken cook on HIGH for 3-4 hours or LOW for 6 hours. If using fresh chicken cook on LOW for 3-4 hours
Once chicken is done cooking, remove it from the slow cooker. Shred chicken with two forks and add back into the slow cooker with the cooking liquid. This will help keep your chicken moist.
To serve: Place chicken in tortilla and finish off with your favorite taco toppings! (Let the extra liquid drain back into the slow cooker a bit before putting chicken in your taco.)