Preheat the oven to 375 degrees F (190 degrees C). Line a baking sheet with aluminum foil; spread beef bones out onto the prepared baking sheet.
Roast bones in the preheated oven until browned, 25 to 30 minutes.
Place carrots, celery, onion, garlic, peppercorns, and bay leaves into a slow cooker. Place roasted bones over vegetables, then pour in enough cold water to cover bones. Add apple cider vinegar and kosher salt.
Cover and cook on Low for 8 hours. Pour broth through a fine-mesh strainer into a bowl and discard any strained solids.