Servings: 2 Quarts

1 ounce (4 packets; about 3 tablespoons) unflavored gelatin 1 ounce (4 packets; about 3 tablespoons) unflavored gelatin
2 pounds skin-on chicken backs and wing tips, preferably from young chickens 2 pounds skin-on chicken backs and wing tips, preferably from young chickens
2 pounds chicken legs 1 2 pounds chicken legs 1
large onion, roughly chopped large onion, roughly chopped
1 large carrot, peeled and roughly chopped 1 large carrot, peeled and roughly chopped
2 stalks celery, roughly chopped 2 stalks celery, roughly chopped
2 bay leaves 2 bay leaves
2 teaspoons whole black peppercorns 2 teaspoons whole black peppercorns
1 teaspoon fennel seeds 1 teaspoon fennel seeds
1 teaspoon coriander seeds 1 teaspoon coriander seeds
6 sprigs fresh parsley 6 sprigs fresh parsley

Pour 4 cups water into a medium bowl and sprinkle with the gelatin. Set aside until the gelatin is hydrated, about 10 minutes.
Meanwhile, chop the chicken backs and wing tips with a cleaver into rough 2-inch pieces, or cut up with poultry shears. Working in two or three batches, transfer the chicken to a food processor and pulse until the texture roughly approximates ground chicken, stopping if a particularly hard bone gets stuck on the blade.
Transfer the ground chicken, chicken legs, onion, carrot, celery, bay leaves, peppercorns, fennel, coriander, and parsley to a large Dutch oven or stockpot and add cold water to just cover the ingredients, about 2 quarts. Add the hydrated gelatin and water and bring to a boil over high heat. Adjust the heat to maintain a brisk simmer, then skim off any foam and scum from the surface and discard. Cook for 45 minutes, adding water as necessary to keep the ingredients submerged. Remove from the heat and allow to cool for a few minutes.
Using a pair of tongs, transfer the chicken legs to a bowl and set aside to cool. Remove and discard any large pieces of bone and vegetables from the stock. Strain the stock through a fine-mesh strainer into a large bowl and discard the solids. Return the stock to the pot, bring to a rolling simmer, and reduce to 2 quarts, about 10 minutes.
Meanwhile, pick the meat off the chicken legs and reserve for another use (such as Chicken Vegetable Soup or Chicken and Dumplings; see here or here). Discard the bones and skin.
When the stock has finished reducing, allow it to rest until the excess fat and scum form a distinct layer on the surface, about 15 minutes, then skim off with a ladle-discard the fat or save it for another use. Alternatively, you can refrigerate the stock overnight and remove the solid fat from the top. The stock will keep in an airtight container in the refrigerator for up to 5 days, or it can be frozen for at least 3 months (see here)