Meanwhile, place the roasting pan on top of the stove and brown the onions, leek, carrots, and celery in the fat in the pan. They should be nicely caramelized; stir often so that they don’t scorch. Add the vegetables to the stockpot. Pour a little water into the pan and simmer, scraping up the browned bits on the bottom. Pour the pan juices into the stock and add the remaining ingredients.
Now simmer the stock uncovered over very low heat as long as possible, preferably all day, but at least 6 to 8 hours.
Remove the solids with a slotted spoon or Chinese skimmer. Pour the stock through a strainer, then line the strainer with cheesecloth and strain the stock again. Let cool, then refrigerate until cold. Remove the layer of fat from the top.
For a richer stock, gently boil the stock uncovered until reduced by half. If you’re using the stock for sauces, reduce it even more. Let cool and refrigerate or freeze.