Heat the oven to 450 degrees. Place the bones on a baking sheet. Roast for 30 minutes, turning the bones occasionally, or until they are well browned.
Remove the bones from the oven and place them in a large stockpot. Pour in enough water to cover the bones. Pour in the apple cider vinegar. Let the bones rest in the vinegar-water for one hour. Add the carrots, celery, garlic, onion, leek, parsley, salt and peppercorns. Add more water to cover the meat and vegetables. Bring the mixture to a boil, lower the heat and skim the surface of any debris that rises to the top for the next several minutes.* Simmer for at least 12 hours and up to 24 hours, skimming the surface occasionally. Strain.
For slow cooker: prepare as above up to * before transferring to a slow cooker for simmering.
Ronnie Fein is a cookbook author, food writer and cooking teacher in Stamford. She is the author of The Modern Kosher Kitchen and Hip Kosher. Visit her food blog, Kitchen Vignettes, at www.ronniefein.com, friend on Facebook at RonnieVailFein, Twitter at @RonnieVFein, Instagram at RonnieVFein.