Dry the duck thoroughly and sprinkle the duck inside and out with salt and pepper.
Use a needle or sharp knife point to pierce the skin of the fat all over, taking care to not pierce the meat itself; go in at an angle. This helps the fat render out of the bird.
Preheat the oven to 160°C.
Put the duck in an ovenproof pan, propping it above the level of the pan. If you want, surround the duck with some root vegetables.
Roast for 90 minutes at 160°C.
Take the pan out and increase the heat to 215°C. When it hits this temperature, put the bird back in the oven and roast until the skin is crispy, about 15 to 20 minutes.
Meanwhile, boil the vinegar and sugar in a small pot until it turns brown. Pour in the stock little by little, stirring constantly and set aside.
Shave the peel off the oranges, grating some fine and keeping the peel of 2 oranges in large pieces. Juice 2 oranges and cut segments from the other 2 oranges.
When the duck is ready, remove it from the oven and let it rest on a cutting board.
Remove as much fat as possible from the roasting pan (reserve this for other uses) and add to the sauce.
Finish the sauce. Bring it to a simmer, then add about ½ cup of orange juice and the large bits of peel. Simmer 5 minutes. Whisk together a little of the sauce with the starch, and, when it's mixed well, stir it into the saucepot to thicken. Add the Grand Marnier (if using) and salt and orange bitters to taste. Swirl in the butter one tablespoon at a time.
To serve, carve the duck and arrange on plate. Add the sauce to a warmed gravy boat. Give everyone some orange segments and pour over the sauce. Garnish with the grated zest.