Dating from the heyday of our infusion experiments, this is a wonderful broth that is deeply colored with the flavors of roasted duck, garlic, and herbal Szechwan peppercorns. It is such a delicious stock that it may be served with nothing more than some softened glass noodles and a sprinkling of scallions, though you can of course embellish it with shreds of duck and vegetables.
If you have never enriched a stock with roasted bones and sweated vegetables, this process will be fascinating. It is also very easy—the kind of thing that can provide a morning’s or evening’s intrigue while you move about the house attending to other chores.
Once made and left to cool, this infusion can be refrigerated for up to a week or frozen for a month.
While we use fresh duck carcasses and bones, you might also use the leftover bones from a roasted duck. Toss them over heat as called for anyway; the additional roasting will bring out their savor.
Roast the garlic until tender, 30 minutes. A bit of black ooze may bubble from the top; don’t worry. Smash the head to break up the cloves.
Heat a wok or large heavy skillet over high heat until hot. Add the corn oil and swirl to glaze the pan. When the oil is nearly smoking, add the duck bones in a single layer (this can be done in batches, if necessary). Brown quickly on all sides, turning the bones once or twice. Reduce the heat to moderate and add the onion, carrots, chili, and scallions. Stir the vegetables until the edges start to curl and brown. Adjust the heat so the vegetables color nicely without scorching.
Transfer the duck bones and vegetables to a non-aluminum, heavy, 6- to 8-quart stockpot. Add the chicken stock and peppercorns, and bring the liquids to a boil over moderate heat. Immediately reduce the heat to maintain a steady simmer. Simmer the infusion for 1 hour. During the last 15 minutes, add the lemongrass.
Strain the infusion through a fine-mesh sieve lined with several layers of dampened cheesecloth. Spoon off any lingering fat.
If using immediately, season with enough kosher salt to bring out the garlic flavor, then end with roasted pepper-salt to taste. For future use, refrigerate the infusion for up to a week or freeze for up to a month. If storing freshly made infusion, allow it first to cool, uncovered, in the refrigerator or at cool room temperature before sealing for storage.