Step 1: Optional: Roast chicken bones in a preheated oven at 400°F (200°C) for 20-30 minutes until browned.
Step 2: Place the bones into a large 12-quart stockpot and cover with 6 quarts of cold filtered water. Add the apple cider vinegar.
Step 3: Bring the mixture to a very gentle simmer over medium-high heat. Do not let it reach a rolling boil.
Step 4: Once simmering, use a spoon or skimmer to remove any gray foam or impurities that rise to the surface.
Step 5: Add the onions, carrots, celery, garlic, parsley, thyme, bay leaves, and peppercorns to the pot.
Step 6: Reduce heat to the lowest setting. Cover partially and simmer for 8-12 hours. Check occasionally to ensure it remains at a bare simmer.
Step 7: Turn off the heat. Strain the stock through a fine-mesh sieve into a clean pot or large heat-proof bowl. Discard solids.
Step 8: Cool the stock quickly in an ice bath. Once cool, refrigerate overnight. The next day, scrape off any solidified fat from the surface if desired.
Step 9: Store in airtight containers in the fridge for up to 5 days or freeze for up to 6 months.