1. Preheat your oven to 400°F (200°C). Place the beef bones on a baking sheet and roast for 30 minutes.
2. Transfer the roasted bones to a large pot. Add the carrots, celery, onion, garlic, apple cider vinegar, salt, and pepper.
3. Pour in the water, making sure the bones and vegetables are fully submerged. Bring to a boil, then reduce to a simmer.
4. Simmer for at least 12 hours, skimming off any foam or fat that rises to the top.
5. Strain the broth through a fine-mesh strainer into a large bowl. Discard the solids.
6. Let the broth cool, then refrigerate. Once chilled, remove any solidified fat from the top.