To prepare the turkey, place it breast side down on a board and, using a very sharp knife, make a deep incision along the centre of the backbone, cutting right down to the bone. Starting at one side, insert the knife under the skin and flesh and, with small cutting strokes, gradually cut the flesh away from the carcass. Keep the knife close to the bone at all times and work from the backbone down to the tip of the breastbone.
When the knife comes into contact with the thigh, work around the ball and socket joint and gently pull it free from the main carcass. Do the same with the wing joint. On reaching the tip of the breastbone, turn the turkey around and repeat along the other side. Finally, work the knife carefully along the tip of the breastbone to free it completely, taking care not to cut through the skin, which is very thin at this point.
To prepare the stuffing, melt half the butter in a large frying pan and cook the chopped onions and celery gently until just softened. Stir in the breadcrumbs and cook for 5 minutes, taking care not to let them burn. Remove from the heat, add the chopped walnuts, marjoram, parsley, orange rind and juice. Season well with salt and pepper, stir in the beaten egg and leave the stuffing to cool.
Preheat the oven to 220°C. Place the turkey, breast side down, on the work surface and open it out flat. Spoon the stuffing along the centre, then bring the sides and neck flap neatly back over the stuffing. Using a trussing needle and fine string, sew the turkey together to enclose the stuffing. Turn the bird over and mould it back into its original shape. Tuck the wings underneath and tie the legs together. Put the prepared turkey into a roasting pan, rub it all over with the remaining butter, then cover it with the streaky bacon. Cover the roasting pan with foil, tucking it securely under the rim of the pan.
Place the pan in the centre of the oven and roast for 50 minutes. Reduce the temperature to 180°C and continue to roast, basting frequently, for a further 1 1/2–2 hours, or until it is cooked through. Remove the foil and bacon for the last 30 minutes of cooking time to allow the bird to brown. Crumble the bacon into small pieces and set aside in a warm place.
To test if the turkey is cooked, pierce the thigh at the thickest part with a skewer. The turkey is ready when the juices run clear. If necessary, return it to the oven and continue roasting until done.
To make the sauce, melt the butter in a saucepan, add the celery and seeds and cook gently until the celery has softened. Remove from the heat and set aside.
Carefully lift the turkey from the roasting pan and put it on a heated serving plate. Cover with foil to rest and keep warm while you finish making the sauce.
Skim all but about 2 tablespoons of the turkey fat from the juices in the roasting pan. Add the flour and stir in, then cook for 1–2 minutes. Add the giblet stock and bring to the boil, stirring and scraping any browned residue off the bottom of the pan. Strain the sauce into the celery mixture and add the sherry. Season to taste with salt and pepper and simmer for 5 minutes.
Garnish the turkey generously with the celery leaves and walnut halves. Scatter over the reserved crispy bacon and serve.