Preheat the oven to 425℉ (220℃).
Arrange the chicken parts, carrots, celery, garlic, and onion in a baking sheet large enough to hold them in one layer, use two baking sheets if necessary. Drizzle oil generously over all the chicken and vegetables. Roast for 45–60 minutes, or until everything is golden and lightly browned, not burned. The veggies should look caramelized, and the pan should have juices and browned bits on the bottom.
vegetable oil, 5 lbs chicken backs, 3 lbs chicken feet, 4 chicken drumsticks, 3 large carrots, 3 celery ribs, 2 large yellow onions, 1 whole head of garlic
Pour all the pan juices into your stockpot. Add a splash of hot water, if needed, to the sheet pans and scrape up every browned bit from the pan and add that to the pot too.
Move the roasted chicken and vegetables into your 18 qt stock pot or two 9 qt Dutch ovens. Add the thyme, bay leaves, peppercorns, cold water to cover, and the apple cider vinegar. The vinegar helps pull collagen from the bones but does not make the broth sour.
5 sprigs fresh thyme, 2 bay leaves, 1 tablespoon whole black peppercorns, 2 tablespoon apple cider vinegar, 16 quarts cold water
Heat the pot over medium heat until you see tiny bubbles around the edges. Keep the heat low. No boiling, just a gentle simmer. During the first hour, skim off any foam, scum, or impurities from the surface with a spoon.
Let the broth cook uncovered for 8-10 hours, or back bones and feet are fully breaking down and falling apart. Add a bit of water while cooking if the bones or vegetables start sticking out of the liquid. Everything should stay covered with water for even cooking.
Pour the broth through a fine-mesh strainer into a large bowl or clean pot. For a super clear broth, run the broth through the strainer and then again through a cheesecloth or coffee filter to catch any tiny veggie/chicken bits.
Taste and season lightly with kosher salt. You'll be using this to make soups, stews, sauces, etc., so it shouldn't be perfectly salty like a soup, just seasoned enough to taste good.
kosher salt
Let it cool, then refrigerate overnight. The fat will rise to the top and can be skimmed off if you prefer. Once chilled, the broth should turn into a gel. Store in the fridge for up to 5 days or freeze in portions for up to a year.