- 6 pounds chicken bones, backs, and necks, and/or bones from whole carcasses
- 2 tablespoons vegetable oil
- 2 medium Spanish onions, peeled, 1 cut into quarters and 1 cut crosswise in half (reserve half for another use)
- 1 large carrot, peeled and cut into 4 pieces
- 1 large leek (3 ounces), roots trimmed, split lengthwise, rinsed well, and cut into 2” pieces, or leek tops
- 1 flat-leaf parsley sprig
- 1 head of garlic, cut horizontally in half (reserve half for another use)
Preheat oven to 450°. Place a large roasting pan in the oven to preheat for about 10 minutes.
Meanwhile, remove and discard the excess fat and any organs from the chicken bones. If there is any skin left on the bones, it does not need to be removed. Rinse bones and dry them well with paper towels.
Add oil to hot roasting pan, then distribute chicken bones in a single layer. Roast bones for 20–30 minutes or until they are well browned on the first side. Turn the bones, add quartered onion and carrot, and continue to roast for another 20–30 minutes or until the bones are well browned but not dried out.
Meanwhile, brown the onion half: Heat a small heavy skillet over medium-high heat for 2–3 minutes. Place onion cut side down on one side of the skillet so that it is not over direct heat and let it brown and char black, about 30 minutes. This will add color to the jus. Set aside.
Transfer the browned bones, onion, and carrot to a colander set over a bowl to drain. (Or, if the vegetables are not well caramelized, leave them in the roasting pan.) Place the pan over medium heat and heat for about 5 minutes to reduce the juices until they cling to the bottom of the pan. (Remove any vegetables as they brown.)
Drain off fat and return roasting pan to the heat. Add 1 cup water to the pan, scrape with a spatula to release the juices on bottom of the pan, and let the liquid reduce by half.
Transfer bones to a large stockpot and add the pan juices. Add cold water (about 3 qts.) to just cover the bones. Any fat in the juices will rise to the top of the pot when the cold water is added; use a skimmer to remove and discard the fat. Add charred onion and a pinch of salt and place pot over medium heat. It is important to bring the liquid to a simmer very slowly to bring up all the impurities. Keep skimming as the impurities rise to the top of the pot. After the liquid has simmered for about 15 minutes, add the leek, bay leaves, peppercorns, thyme, parsley, and garlic. Continue to simmer for 2 3/4 hours. The jus will reduce to about 1–2" below the level of the bones; do not add additional water to the pot.
Line a strainer with cheesecloth and place over a large bowl. Removing the bones from the pot or pouring out the contents will cloud the jus. Instead, carefully ladle the jus out of the pot and pass it through the strainer, tilting the pot as necessary to get all the jus. (There will be about 2 qts.) (At this point, the jus can be used for poaching or as a soup base or stock. Cool in an ice bath, then refrigerate.)
To finish the jus, pour it into a saucepan and simmer it to reduce to 4 cups. Strain once more through a strainer lined with cheesecloth and cool in an ice bath. (Store the jus in the refrigerator for up to 3 days, or freeze for longer storage.)