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Rinse the chicken backs and necks well under running water for 5 minutes. Place the backs and necks in a 20-quart stockpot, top with the ice, and bring to a simmer over medium heat. Simmer for 15 to 20 minutes, skimming off all of the impurities and fats that rise to the top. After the stock is skimmed, add the leek, celery, celery root, shallot, and fennel. Make a sachet by wrapping the peppercorns, bay leaf, and thyme in cheesecloth. Add the sachet to the stock. Simmer, uncovered, for 3 hours, skimming every 30 minutes. Strain through a chinois and chill over an ice bath. Store in an airtight container in the refrigerator for up to 1 week or freeze in ice cube trays for up to 1 month.
