Remove the duckling from the oven and keep warm. Pour off all but two tablespoons of fat from the roasting pan, add the onions and celery, and saute until tender. Add two cups of brown sauce. Boil for 20 minutes. The basic sauce is now ready for additional flavoring. STEP TWO: Prepare the Orange Sauce-Melt the butter, vinegar, and sugar in a saucepan and boil until the liquid caramelizes. Add the orange juice, mustard, and basic brown sauce and bring to a boil. Strain the sauce, then thicken with cornstarch and season with Grand Marnier.