Up to four days before, make the brine by combining the salt, peppercorns, bay, sage and sugar using a pestle and mortar, or crush with the end of a rolling pin in a bowl. Stir through both zests and spoon roughly 1/ 3 into a large resealable bag and the rest in another bag to chill.
Put the turkey legs and wings in the bag with 1/ 3 of the brine and shake well to coat the skin, then leave overnight in the fridge.
The next day, heat oven to 120C/ 100C fan/gas 1. Rinse the brine from the legs and wings and pat dry. Put the duck fat in a roasting tin, melt in the oven for 5 mins, then add the turkey pieces (they should be submerged in the fat). Cook in the middle of the oven for about 4 hrs or until the legs are tender. Leave to cool, then cover and chill overnight, or up to three days.
On Christmas Eve, cover the turkey crown in the remaining dry brine and chill in the fridge overnight.
In the morning, take the crown out the fridge 1 hr before roasting, rinse off the brine and pat dry with kitchen paper. Take the confit legs, thighs and wings out the fridge and leave to come to room temperature so they are easily released from the fat. Take out and put on a baking tray ready to reheat later. Strain the fat and set aside ready to cook the potatoes.
Heat oven to 190C/170C fan/gas 5 and line a large shallow roasting tin with foil. Set a wire rack on top, scatter the herbs and onion across and sit the crown on top. Rub the butter all over the skin and season well. Roast, uncovered, for 40 mins, then cover with foil and cook for another 30 mins. Remove the foil, and cook for a final 15-20 mins. To test if the turkey is cooked, insert a skewer into the thickest part of the breast – the juices should run clear. Transfer to a platter, cover with foil and leave to rest for at least 30 mins.
Meanwhile, reheat the confit legs and wings in the oven for 30 mins or until the skin has crisped up. Add to the platter with the crown and carve.