In large stockpot or Dutch oven, bring chicken and 12 cups water to boil; skim off froth. Add salt and pepper; reduce heat and simmer gently for 1 hour.
Meanwhile, peel carrots, parsnips and parsley root; cut into large chunks along with celery. Add to pot along with onion, parsley sprigs and garlic. simmer for 1 hour. Remove chicken; refrigerate for another use. Strain liquid through cheesecloth-lined sieve, pressing down on vegetables to extract as much liquid as possible. Let cool to room temperature.
cover and refrigerate for at least 8 hours or until fat has congealed on surface. Remove fat with slotted spoon. [Can be refrigerated for up to 3 days or frozen for up to 3 months.]
Add-Ins: Add shredded cooked chicken Add 4 cups cooked noodles, cooked rice, or matzo balls Reserve carrots and celery from stock; slice and add to soup.
