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 or slow cooker. Add carrots, celery, onion, and garlic.
03 - Pour in cold water until bones are fully submerged. Add apple cider vinegar, bay leaves, peppercorns, thyme, and parsley.
04 - Bring to a gentle boil over medium-high heat. Skim off any foam or impurities that rise to the surface.
05 - Reduce heat to low and simmer uncovered for 12 hours, adding water as needed to keep bones covered.
06 - In the final hour of simmering, season with sea salt to taste.
07 - Strain broth through a fine-mesh sieve into a clean container, discarding solids.
08 - Allow broth to cool, then refrigerate. Once chilled, skim off solidified fat if desired. Store broth in the refrigerator up to 5 days or freeze for longer shelf life.