Preheat the oven to 400°F. Toss the chicken bones with the salt, arrange them in a single layer in a roasting pan, and roast until golden brown, about 30 minutes.

While the bones are roasting, heat a frying pan over medium-high heat. Add the onions, cut side down, and sear until they’re dark brown. Remove them from the pan and chop.

Place the roasted bones and onions in a stockpot with the garlic, carrots, onions, celery, leeks, thyme, bay leaves, and peppercorns. Add just enough water to cover the bones and bring to a simmer, uncovered, over medium-high heat. Turn down the heat to low, use a spoon to skim off any scum that’s risen to the top of the stock, and simmer for another 2 hours.

Remove the stock from the heat and allow to cool for 1 hour before straining through a fine-mesh sieve into a clean container, like a mason jar. Refrigerate for up to 1 week or freeze for up to 3 months.