Preheat oven to 200 C / Gas 6. Line a baking tray with foil; spread beef bones out on prepared baking tray.
Roast bones in the preheated oven until browned, 25 to 30 minutes.
Place carrots, celery, onion, garlic, peppercorns and bay leaves in a slow cooker. Place roasted bones over vegetables; pour in enough cold water to cover bones. Add apple cider vinegar and sea salt.
Cook on Low for 8 hours. Pour broth through a fine sieve into a bowl and discard any strained solids.
You can skim the froth that forms at the surface of this broth as it cooks, but there's no real reason to do so. Do this only if you must have a clear broth. Otherwise, keep it in and keep the health benefits.