Combine the shallots and vinegar in a large saucepan and bring to a boil. Cook until the liquid is reduced by half. Add the chicken stock and reduce again by half. Add the cream and cook, over medium-low heat, reducing the liquid until it reaches a sauce consistency, about 15 to 20 minutes. Remove the sauce from the heat and whisk in the mustard, salt and pepper.
Yield: about 1 1/4 cups