Preheat the oven to 350. Roast the bones for 1 hour in cast iron pans, or alternately, baking sheets lined with parchment for easy cleanup can be used.
Drain the fat from the pans and reserve for another use or discard. Put the bones in a slow cooker with the vegetables, top off the pot with water, and set on low for 24 hours, and as long as 48, topping off with fresh water if the level gets far below the bones for periods of extended cooking.
Remove the bones, strain the stock, season to taste with salt, then chill completely to set the fat. Skim off the fat from the chilled broth, then refrigerate for up to 4 days, or freeze until needed. Chef Bergo prefers to freeze his, since thawing in a pan is quick and easy.
To serve the bone broth, heat some of the broth, seasoning with a few cracks of the pepper mill and a dash of fresh lemon juice. Put a small rosemary sprig into a mug for each person, then pour steaming hot broth into the mugs and serve.