Layer the bones in the bottom of the slow cooker, then top with the carrots, celery, onion, garlic, salt, peppercorns, apple cider vinegar, and bay leaves (if using).
Fill the slow cooker with water, leaving some space at the top to prevent spills.
Cover and cook on low for 18-48 hours (up to 72 hours if you want an extra rich broth). Remove the lid and cool for 15 to 20 minutes, then place the slow cooker bowl in the refrigerator to chill until the fat rises to the top and solidifies a bit (this will take a few hours).
Using a ladle or large spoon, skim the fat from the top of the broth (you might not be able to get all of it in this step). Using tongs, remove and discard the bones and vegetables, then strain the broth through a fine mesh strainer lined with cheesecloth (preferably 2 layers) to catch the remaining fat and scum.
Heat the bone broth and enjoy! Bone broth will keep in the fridge for 3 days, or in the freezer for up to 3 months.
Try to vegetables use that are organic and bones that are free of hormones and antibiotics. See post for more info on this.For the best possible broth flavor, avoid marrow bones.The quantities in this recipe can be adjusted based on the size of your slow cooker, and you can go heavier on the bones for additional collagen. The vegetable quantities are flexible as well. More veggies and aromatics will mean more flavor.