Preheat the oven to 190C/170C Fan/Gas 5.
For the spinach layer, heat the oil in a pan, add the onion and fry until soft. Add the garlic and cook through. Add the spinach and cooked until wilted. Remove from the heat and press to squeeze out and discard some of the spinach juices.
Place the spinach in a blender with the mascarpone, grated nutmeg and salt and pepper. Blend well and allow to cool completely.
For the stuffing, heat the oil and fry the shallots over a low heat until lightly golden-brown. Add the mushrooms and cook through before adding the sage, parsley and garlic. Cook for a further 2–3 minutes. Pour in the wine and season with salt and pepper. Stir in the apricots and breadcrumbs and add enough chicken stock to bind together. Set aside to cool.
To prepare the turkey, cover the breast with a large piece of cling film and flatten it into an even thickness by hitting it with a mallet or rolling pin. Discard the cling film and season the turkey with salt and pepper.
Spread the spinach layer, followed by the pancetta (or ham) and then the stuffing. Roll up and secure with kitchen string.
At this point weigh the turkey piece to calculate the cooking time: allow 20 minutes per 500g/1lb 2oz, plus an additional 20 minutes at the end.
For the basting liquid, melt the butter, add the cider and heat through. Season with salt and pepper.
Place the turkey in a roasting tray and baste well before transferring to the oven. Continue to baste from time to time, but cover with kitchen foil if it’s browning too much. Roast for the calculated time, or until the juices in the thickest part of the breast run clear (if using a meat thermometer, the stuffed turkey should reach a core temperature of 72C). Transfer the turkey to a plate and cover to keep warm while you let it rest. Retain the tray with the cooking juices.
For the gravy, make a roux by melting the butter in a saucepan, adding the flour and cooking until the mixture whitens a little.
Place the turkey roasting tray over the heat, add the shallots and cook over a medium heat until soft (about 3 minutes). Add the sage and the brandy (or calvados) and simmer to burn off the alcohol.
Pour in the chicken stock, return to a boil, and cook until the volume of liquid reduces a little. Add some of the roux to thicken the sauce. Pour in the cream and allow to simmer for a minute.
To garnish, add the butter to a frying pan set over a medium heat and allow to melt. Place the apples in the pan cut-side down. Sprinkle over the sugar and sage and fry until glazed.
Remove the string from the turkey and serve it in slices with the cider gravy and glazed apples. Garnish with rosemary.