Preheat oven to 425 degrees and line a baking sheet with parchment paper.
Arrange beef bones, onion, carrots, leek, and garlic on the prepared baking sheet and roast until beef bones are browned, about 30 minutes.
Place celery, bay leaves, parsley, rosemary, thyme, and peppercorns in slow cooker followed by the roasted bones and vegetables. Pour in apple cider vinegar and fill slow cooker with filtered water until all the contents are fully submerged. Secure the lid and cook on low for 8 to 12 hours. The longer the broth cooks, the richer the flavor will be. During the cooking process, if too much liquid evaporates, add a bit more water.
Using a fine mesh sieve, drain the broth and discard the solids. Broth can be stored in the fridge for up to 5 days or frozen for later use.