Roast chicken bones
Preheat the oven to 400ºF (200ºC). Arrange chicken bones in an even single layer on a baking sheet. Roast bones until browned, 1 hour.
Add bones and vegetables
Add the bones to a large stock pot or large Dutch oven. Add the onion, celery, carrot and garlic.
Cover with water
Cover with water (about 4 quarts of water) and add the parsley, bay leaves and peppercorns.
Simmer the stock
Bring to a boil over medium heat then lower the temperature and simmer, using a spoon to skim off any scum that rises to the surface and discard it.
Skim any scum
Continue to keep at a gentle simmer (just a few small bubbles) for about 6 hours, until reduced by about half. Add additional water as needed.
Once the stock is done, remove from heat and let cool. Use tongs to remove any large bones, then strain the stock through a fine mesh strainer into a large bowl, discarding solids.
Let cool for about an hour then transfer the broth to storage containers or resealable plastic bags and refrigerate or freeze until ready to use.