01 - Preheat oven to 425°F. Arrange beef bones on a roasting pan and roast for 30 minutes, turning once until deeply browned.
02 - Transfer roasted bones to a large stockpot. Add carrots, celery, onion, and garlic.
03 - Pour in apple cider vinegar and cold water to cover the bones and vegetables. Add bay leaves, peppercorns, parsley, and thyme.
04 - Bring to a gentle simmer over medium heat. Skim off any foam or impurities during the first 30 minutes.
05 - Reduce heat to low and simmer uncovered for 8 hours, adding water as needed to keep bones submerged.
06 - Season broth with salt. Strain through a fine-mesh sieve into a large bowl, discarding solids.
07 - Cool to room temperature, refrigerate, and once chilled, skim off fat if desired. Store in airtight containers in the refrigerator for up to 5 days or freeze up to 3 months.