Place the whole chicken and 12 cups of cold water in a large stockpot or Dutch oven. Bring it to a boil over high heat, then immediately reduce the heat to low to maintain a gentle simmer.
Use a large spoon to skim off and discard any foam or impurities that rise to the surface during the first 10 minutes of simmering.
Add the chopped onion, sliced carrots, sliced celery, 2 teaspoons of kosher salt, whole peppercorns, and bay leaves to the pot. Let the soup simmer gently, partially covered, for 1 hour and 15 minutes.
Carefully remove the chicken from the pot and place it on a large cutting board. The chicken should be very tender and the meat should easily pull away from the bone.
Use two forks to shred all the meat from the bones, discarding the skin and bones. Return the shredded chicken meat to the pot.
Stir in the chopped fresh parsley and dill. Taste the soup and season with additional salt if needed. Simmer for 5 more minutes to allow the flavors to combine before serving.
