Place the chicken in a large pot. Cut 3 of the carrots and 2 of the celery stalks into 1-inch pieces. Quarter the onion. Add the cut vegetables to the pot with the salt, peppercorns, and enough cold water to cover (about 8 cups). Bring to a boil. Reduce heat and simmer, skimming any foam that rises to the top, until the chicken is cooked through, about 30 minutes.
Transfer the chicken to a bowl and let cool. Strain the broth, discarding the vegetables. Return the broth to the pot. Thinly slice the remaining carrots and celery. Add them to the broth and simmer until tender, about 10 minutes. When the chicken is cool enough to handle, shred the meat and add it to the soup. Ladle into individual bowls.