Prepare the Duckling and Basic Brown Sauce.
Trim the duck of excess fat at the base of the tail. Rub the inside with the seasoning salt and rosemary.
Preheat oven to 450 degrees F.
Place the duckling in a roasting pan, add 1/2 cup of water, and roast for 1-1/2 hours, basting occasionally.
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 sauté until tender. Add two cups of brown sauce. Boil for 20 minutes.
The basic sauce is now ready for additional flavoring.
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.