This recipe is from Michael Field's Cooking School and should accompany the Marinated Leg of Lamb.
1. In the top of a double boiler, whisk the ingredients through cayene pepper together until warm. Add the chicken stock and whisk vigorously for about 2 minutes until hot, when the arrowroot will thicken the sauce. Add the parsley. The sauce will seem thin when hot but thickens when merely warm.