01 - Place the beef bones in a large stockpot or slow cooker. For a deeper, richer flavor, roast the bones beforehand at 400°F for 30 minutes until nicely browned.
02 - Add the roughly chopped carrots, celery, quartered onion, smashed garlic, bay leaves, fresh parsley, and whole peppercorns to the pot with the bones.
03 - Pour in the apple cider vinegar and cold water, making sure the bones and vegetables are completely submerged.
04 - Bring the mixture to a gentle simmer over medium heat. During the first hour, use a skimmer or ladle to remove any foam or impurities that rise to the surface.
05 - Reduce the heat to low and let the broth simmer uncovered or partially covered for at least 12 hours. Check periodically and add water as needed to keep the bones submerged.
06 - Strain the broth through a fine-mesh sieve or cheesecloth into a clean container. Discard all solids. Season the broth with salt to taste.
07 - Allow the broth to cool to room temperature, then refrigerate. Once chilled, the fat will solidify on the surface and can be skimmed off if desired. Store refrigerated for up to 5 days or freeze for up to 3 months.