Bring bones and water to a boil in an 8 quart pot. Skim off foam/scum that rises to the top and discard. Add onions, carrots, celery, garlic, parsley, apple cider vinegar and peppercorns. Bring back up to a boil, then lower flame and simmer 6-18 hours, covered. The longer you cook the stock the more concentrated it becomes.