Place the chicken breasts in a medium saucepan or Dutch oven. Add enough chicken stock to cover the breasts and bring to a boil. Lower to a simmer and cook for 30 minutes. Remove the chicken from the stock and shred it using two forks (or your hands). Cover and set aside until ready to use.
Meanwhile, heat the olive oil in a large heavy-bottomed saucepan over medium heat. When hot add the carrots, celery, onion, and garlic and cook for 5 minutes until the onions are translucent and the vegetables are just tender.
Add the sweet potato and cayenne pepper and cook for 1 minute. Strain the chicken stock used to cook the chicken breasts into the pot and add the remaining stock (i.e. whatever you did not use to cook the chicken). Bring to a boil, then lower to a simmer and cook for 15 minutes until the sweet potatoes are just tender.
Add the shredded chicken and quinoa to the pot and simmer for another 20-25 minutes. Season with salt and pepper to taste.
Serve your soup piping hot garnished with cilantro and avocado.