Preheat the oven to 250 ° C. Thoroughly wash the duck outside and inside, rub with salt and pepper. Fry the duck in a roaster WITHOUT fat for about 30 minutes until crispy and brown (turn once). In the meantime, brush off the oranges and cut the outer layer of peel into the finest strips with zester. Squeeze oranges.
Just bring the sherry vinegar to the boil, then sprinkle with sugar and caramelize over high heat (but be careful not to turn it black). Reduce the oven heat to 180 ° C (circulating air 150 ° C).
Take the duck out of the roasting pan and pour off the stock into a separate container. Put a little orange juice on the bottom of the roaster. Put the duck back (breast up) and spread the orange peel strips on the duck. Pour caramel over the duck and put it back in the oven. Roast the duck for approx. 1 hour and baste it with orange juice and frying stock alternately every 10 minutes.
Then carve the duck on a preheated platter, heat the Grand Marnier in a saucepan, light it and flambé the duck with it. Keep warm again. In the meantime, bring the sauce that has been poured out of the roaster to a boil and add the port wine, let it boil for approx. 5 minutes, whisk through with a whisk. Serve the duck in portions on plates, nappate generously with the sauce (pour over).
Serve with small potato pancakes or french fries duchesse.