Preheat the oven to 450°F. Place beef bones, carrots, leek, onion, and garlic on a roasting pan or rimmed baking sheet, and roast for 20 minutes. Toss every 5 minutes or so until vegetables reach a deep brown color.
Fill a large stockpot with 12 cups of water. Add celery, bay leaves, and peppercorns. Scrape the roasted bones and vegetables into the pot along with any juices. Add more water if necessary to cover bones and vegetables entirely.
Cover the pot and bring to a gentle boil. Reduce heat to a very low simmer (with lid on and tilted) for 10 hours. Occasionally skim foam and excess fat. Add more water if necessary to keep everything submerged).
Remove the pot from the heat and let cool slightly. Strain broth using a fine-mesh sieve and discard bones and vegetables. Let continue to cool until barely warm, then refrigerate in smaller containers overnight. Remove solidified fat from the top of the chilled broth.