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Preheat the oven to fan 160C/conventional180C/gas 4. Cut the parsnips in half widthways and the fatter ends into four so that all the pieces are roughly the same size. Put 2 tbsp of the oil in a small roasting tin, toss in the parsnips and roast for 20 minutes.
Meanwhile, cut the pork fillet in half to make two shorter pieces, then slice through each piece – but not all the way through – so you can open it like a book. Cut the cheese in two, put a slice on one half of each piece of pork and sprinkle with the chopped sage and season. Fold the pork over – closing the book – to sandwich the cheese, and tie together with string to make two parcels. Season the pork on both sides.
Heat the remaining tablespoon of oil with one knob of the butter in a frying pan until sizzling, then brown the pork for 2-3 minutes on each side.When the parsnips have had their 20 minutes cooking time, put the pork on top of them and return to the oven for 15 minutes.This will give you slightly pink pork – if you prefer it to be well done, cook it for 20-25 minutes.
While the pork is roasting, heat another knob of butter in a frying pan and fry the sage leaves for about 30 seconds until crisp, then put them on a plate.Now add the apples to the pan and cook over a fairly high heat until caramelized. Lift them out, then add the cabbage to the pan and stir fry until tender.
Spoon the cabbage onto two plates. Snip the string from the pork and discard. Put the pork on top of the cabbage, then arrange the parsnips and apples on either side. Put the roasting tin on high heat on the stove and tip in the wine or cider. Bubble for 3-4 minutes to reduce and then pour over the meat. Scatter the frizzled sage leaves on top and serve.