turkey gravy recipe

2 tablespoons unsalted butter 1 medium onion (or leek, or 2 shallots, sliced) Neck and giblets from your turkey (discard the liver) 8 cups low-sodium chicken broth 3 sprigs thyme (or parsley, rosemary, and/or sage) 1 bay leaf Turkey drippings from your roasting pan ½ cup all-purpose flour Dash of Worcestershire sauce Kosher salt and freshly ground pepper 2 tablespoons cold flavored butter (optional)