In a large pot add 4 quarts of chicken broth, onions, 1/4 of total celery, 1/4 of total carrots, salt and pepper.
Remove plastic thermostat and skin from chicken, trim any excess fat then rinse well. Place thyme sprigs inside chicken then place in pot with broth mixture.
Bring pot to boil and reduce heat to low boil and cover. Cook until chicken falls the bones. About 45 minutes to an hour.
Remove from heat and remove bones from broth.
Add remaining 2 quarts of broth. Add remaining carrots and celery along with leeks and parsnips. Bring to a boil then reduce heat to a simmer and cover, gently stirring occasionally. Cook until vegetables are tender. About 30 minutes.