Line a rimmed baking sheet with parchment paper and place bones, onion, carrots, celery and garlic in a single layer. Roast for 40 minutes.
Transfer roasted bones and vegetables to slow cooker. Add peppercorns, vinegar and 16 cups water (filtered if possible). Cook on low for 17 hours (or up to 24 hours).
Allow to cool, then strain solids from slow cooker and transfer bone broth to air tight containers. Refrigerate overnight then skim fat from the top of the cold broth. If you plan to use the broth within a few days, leave it in the refrigerator. Otherwise, transfer to freezer to store for up to 6 months.