1Bring the chicken stock to a simmer. Add the onion, carrot, celery, garlic and chile, and simmer 15 minutes.
2Add the tomatoes, and simmer for another five minutes. Taste and adjust salt, then stir in the chicken breast, cumin and chopped cilantro. Cover and let sit for five minutes. Add the lime juice.
3Divide the avocado between four soup bowls. Ladle in the soup, place a slice of lime on each bowl, and sprinkle crispy tortilla chips and a generous dollop (about 1/8 sour cream) on top of each serving. Serve immediately.