Chop reserved turkey neck, gizzards, and trimmings into 1-inch chunks with a cleaver. Heat oil in a medium saucepan over medium-high heat until shimmering. Add turkey parts, onion, carrot, and celery and cook, stirring occasionally, until well browned, about 10 minutes total. Add stock, soy sauce, Marmite , herbs, and bay leaves. Bring to a boil, then reduce to a simmer. Simmer for 1 hour, then strain through a fine-mesh strainer. Add any drippings from the roast turkey, then skim off excess fat.
