Preheat the oven to 425°F.
In a large bowl, toss the celery, carrots, onion, and garlic with the melted ghee. Put the bones in a roasting pan, add the vegetables, and spread everything into an even layer. Roast at 425°F for 15 to 20 minutes, or until the vegetables start to brown—just don’t let anything burn! Remove from the oven and transfer to a 6-quart slow cooker.
Add 12 cups of water to the slow cooker (reduce the amount if your slow cooker is smaller). Add bay leaves, salt, pepper, and apple cider vinegar. Cover and cook on Low for 12 hours.
While the broth is hot, strain out the solids and skim off any fat from the surface. Let cool. You may notice it turns a bit jelly-like in the fridge—don’t panic! That’s the collagen doing its magic. When reheating, bring the broth to a boil and give it a good stir to reincorporate.