Step 1: Place the chicken bones and carcasses into a very large stockpot (at least 12-quart capacity).
Step 2: Cover the bones with 6 quarts of cold filtered water, ensuring there is at least 2 inches of liquid above the bones.
Step 3: Place the pot over medium-high heat and bring it just to a simmer. Once it starts bubbling gently, reduce the heat to low.
Step 4: Use a wide spoon or a skimmer to remove the gray foam (impurities) that rises to the top during the first 30 minutes.
Step 5: Add the onions, carrots, celery, garlic, thyme, bay leaves, and peppercorns to the pot.
Step 6: Maintain a very low simmer (the surface should barely move) for at least 4 hours. Do not cover the pot completely; leave the lid slightly ajar.
Step 7: Carefully strain the stock through a fine-mesh sieve into a large heat-proof bowl or second pot, discarding the solids.
Step 8: Cool the stock quickly in an ice bath, then transfer to storage containers and refrigerate or freeze.