Marinade: Mix the parsley root, carrot, onion and spices in a bowl then drizzle with white wine. Stir everything to combine.
Place the duck in another bowl and pour the marinade over it. Cover with foil and allow to marinade in the fridge for at least 12 hours.
Stuffing: Mix the chestnuts, toast, apple pieces and egg then season with salt, pepper and dried tarragon.
Preheat the oven to 220 degrees C.
Remove the duck from the marinade and pour the marinade through a sieve. Dry the duck inside and out with paper towel and rub with salt and pepper.
Fill in the duck with the stuffing and close the cavity with skewers. Pour the vegetable oil into a roasting dish then place the duck in it breast up and roast in the preheated oven for 20 minutes until golden-brown.
Heat the marinade in a saucepan and pour into the duck's roasting dish and put a lid on or cover with alfoil.
Reduce the oven temperature to 180 degrees C and cook the duck for a further 90 minutes.
Remove the duck from the dish and place on a baking tray. Turn on the grill and grill it for a few minutes at a high temperature to make the skin golden-brown.
Remove from the oven then cover the duck with foil and keep warm.
Sieve the stock to remove any bits then skim off the fat. Heat the stock over high heat until 1/3 remains and season with salt and pepper. Carve the duck and serve with the gravy.