Place the chicken breasts in the bottom of the slow cooker. Add the diced onion, minced garlic, black beans, diced tomatoes, chicken broth, corn, chili powder, cumin, and salt.
Stir the ingredients gently to combine, then cover and cook on low for 6 to 7 hours until the chicken is fully cooked and tender.
Remove the chicken breasts from the slow cooker and shred them with two forks. Return the shredded chicken to the pot.
Stir in the lime juice. Let the soup sit for 5 minutes so the flavors blend.
Ladle the soup into bowls and top each serving with cilantro, avocado slices, and baked tortilla strips.