Place the bones in a single layer on a baking sheet or roasting pan; roast for 60 minutes, flipping each bone over half way through cooking time.
Place chopped vegetables (don't even bother peeling them), bay leaf, and vinegar in 7-quart slow cooker. Add roasted bones and cover completely with water.
Cook on LOW for 18-36 hours, until deep brown and aromatic.
Carefully remove the larger pieces with a slotted spoon and place them in a colander set over a large bowl to collect the excess broth. Then, strain the broth through a fine meshed sieve, or through a strainer that's been lined with cheesecloth.
Transfer the bone broth into jars and refrigerate promptly. When cooled, remove fat layer and save it for cooking.
Reheat before serving and season to taste.
Notes
Don't peel vegetables—skins add nutrients and color
Save the rendered fat layer for cooking; it's incredibly flavorful
7-quart slow cooker essential for proper water coverage
Properly made broth will gel when refrigerated
Store up to 7 days refrigerated or 6 months frozen
For clearest broth, strain twice through fine-mesh sieve