Preheat oven to 500º F.

Roughly chop the orange rinds after juicing and place in the cleaned duck cavity. Place the stuffed duck on a baking rack inside a roasting pan with 1/2-inch of water. Bake until skin turns golden brown and lightly crisped, about 30 minutes. Reduce temperature to 350º F and continue cooking until duck reaches an internal temperature of 170ºF about 1 1/2 hours.

In a medium heavy saucepan combine the orange juice, zest and sugar over medium high heat and reduce nearly 3/4 in volume, about 3/4 cup. Add Peychaud Bitters to orange juice gastrique and set aside. Place duck stock in a clean saucepan and bring to a boil over high heat. Add hot stock to reduced orange gastrique and continue to simmer over medium low heat for 10 minutes to make sauce.

Remove duck and the rack from the roasting pan and discard drippings from the bottom of pan. Return duck to roasting pan and set over 2 burners over medium high heat. Add Grand Marnier to the pan and cook off the alcohol, scraping the pan continuously with a large wooden spoon. Add the orange sauce to the roasting pan along with the slurry and cook 1 to 2 minutes until thickened. Brush the duck with the sauce, then remove duck from the pan and discard orange rinds in cavity. Place the duck on a serving platter and let sit 10 minutes before carving. Add the pan sauce to a gravy boat and serve with the carved duck.