01 - Place the beef bones in a large stockpot or slow cooker. For deeper flavor, roast bones beforehand at 400°F for 30 minutes until well browned.
02 - Add the roughly chopped carrots, celery, quartered onion, smashed garlic, bay leaves, parsley sprigs, and whole peppercorns to the pot with the bones.
03 - Pour in the apple cider vinegar and 12 cups of cold water, ensuring all bones and vegetables are fully 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 heat to low and simmer uncovered or partially covered for at least 12 hours. Check periodically and add water as needed to keep the bones submerged throughout the cooking process.
06 - Strain the finished broth through a fine-mesh sieve or cheesecloth into a clean container. Discard all solids. Season the strained 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 easily skimmed off if desired. Store refrigerated for up to 5 days or freeze for up to 3 months.