Bring the chicken stock to a simmer, add the onion, garlic and chile, and simmer 15 minutes. Add the tomatoes, and simmer for another five minutes. Taste and adjust salt, then stir in the chicken breast and chopped cilantro. Cover and let sit for five minutes. Add the lime juice. Divide the avocado between four soup bowls. Ladle in the soup, place a slice of lime on each bowl, and serve with crisp corn tortillas.