In a large slow cooker, whisk together the broth, taco seasoning, cumin, and garlic powder. Add the remaining ingredients and stir to combine.
Cover and cook on LOW for 4-6 hours or on HIGH for 2-3 hours, until the chicken is cooked through and the onions are soft. Shred the chicken with two forks; stir the meat back into the soup. Taste and season with salt and pepper, if desired.
Ladle into bowls and garnish with optional toppings.
