Heat a medium pan over medium-high heat, add oil and onions and sauté until browned, about 5 minutes. Set aside.
Assemble ingredients in slow cooker, starting with the chicken breasts, diced tomatoes, corn, beans, strained tomatoes, chicken broth, water, lime juice, spices, sautéed onions, and half of the chopped cilantro.
Set the slow cooker to high for 3 hours, giving it a gentle stir once every hour.
When chicken is fully cooked, remove from the slow cooker and use forks to shred it.
Add chicken back to the soup and stir to combine.
Serve the soup topped with sliced avocado, cilantro, and tortilla strips as garnish.