Poach the chicken: Place the chicken breast, onion half (halved again), salt, pepper and garlic into a medium saucepan. Add enough cold water to cover the chicken. Bring to a boil over high heat and lower heat to a simmer. Simmer chicken for 25 minutes. Remove from heat and allow chicken to sit in the liquid until cool enough to handle.
In a large pot, warm the oil over medium heat. Add the chopped onion, scallions, carrots, celery and jalapeno. Sauté until onions are tender, about 5 minutes. Add garlic and cumin and cook for 1 minute.
Add the chicken stock, salt and pepper and bring to a boil. Lower heat and simmer until carrots are tender, about 4-5 minutes.
Pull chicken from liquid and remove skin and bones. Shred chicken into bite-size pieces. You can divide chicken amongst the 6 individual bowls or add it all to the pot.
Put a heaping spoonful of pico de gallo and avocado chunks/guacamole in each individual bowl and ladle soup on top. Squeeze lime on top and sprinkle with corn tortilla chips and fresh cilantro, if desired.