01 - Place the chicken bones into a 6–8 quart slow cooker, spreading them evenly across the bottom.
02 - Add the roughly chopped carrots, celery, quartered onion, and smashed garlic cloves on top of the bones.
03 - Tuck in the bay leaves, whole peppercorns, fresh parsley, and thyme sprigs among the bones and vegetables.
04 - Pour the apple cider vinegar and cold water over all the ingredients, ensuring everything is fully submerged.
05 - Cover and cook on LOW for 12 to 18 hours, skimming off any foam that rises to the surface as needed throughout the cooking process.
06 - Carefully strain the broth through a fine-mesh sieve or cheesecloth into a large bowl. Discard all spent solids.
07 - Season the strained broth with salt to taste. Allow it to cool, then skim off and discard any solidified fat from the surface if desired.
08 - Transfer the finished broth to airtight containers or jars. Refrigerate for up to 5 days, or freeze for longer storage.