Heat olive oil in a large pot over medium heat. Add diced onion and minced garlic, sautéing for 3-4 minutes until onion becomes translucent and fragrant.
Add whole chicken breasts, black beans, corn, enchilada sauce, diced tomatoes, chicken broth, chili powder, and cumin to the pot. Season with salt and pepper and stir to combine.
Bring mixture to a rolling boil, then reduce heat to medium-low. Simmer gently for 20-25 minutes until chicken is cooked through and reaches 165°F internally.
Remove chicken breasts with tongs and shred with two forks while still warm. Return shredded chicken to the pot and stir to distribute evenly.
Taste and adjust seasoning. Ladle hot soup into bowls and top with shredded cheese, sour cream, cilantro, avocado slices, lime wedges, and tortilla strips as desired.
Notes
Use low-sodium chicken broth to control salt levels. Rinse black beans to remove excess sodium.
Shred chicken while warm for easier handling. Can substitute rotisserie chicken and add during last 10 minutes.
Soup tastes better the next day. Freezes well for up to 3 months in airtight containers.
Check that enchilada sauce and broth are gluten-free if needed, as some brands contain wheat thickeners.