Line a baking sheet with baking paper. Reserve a little of the beaten egg in a small bowl to glaze the pastry.
Heat a frying pan and add the oil. Fry the onion and apple over a high heat for a few minutes. Lower the heat, cover the pan and leave to cook for about 15 minutes until softened. Remove the pan from the heat, transfer the onion and apple to a bowl and leave to cool.
Add the sage, breadcrumbs, Stilton, mustard and the remaining egg to the onion and apple, season well and stir.
Place the pork on a board. Cover it with baking paper and bash with a wooden rolling pin in the thickest areas, until the ﬁllet is the same thickness throughout. Keep it in a circular shape.
Season the meat and spoon the ﬁlling mixture on top, spreading it out to cover the pork ﬁllet completely.
Unroll the pastry and lay it on a floured work surface. Roll it out to make a rectangle that is big enough to encase the ﬁllet. Place the pastry on the baking sheet.
Put the ﬁllet in the centre of the pastry and brush the pastry around the pork with egg. Slice the pastry extending from the pork into 2cm (3∕4in) strips. Fold these over to make a plait on top – just like you would plait hair – then trim away any extra leftover pastry. Chill for 30 minutes.
Preheat the oven to 220ºC/200ºC fan/Gas 7. Brush the plait with egg, then bake for 35-40 minutes.
Cover the plait with foil if it is getting too brown. Rest for 10 minutes.
Meanwhile, make the sauce. Pour the wine into a large pan and reduce by half over a high heat. Pour in the cream, bring to the boil and reduce a little. Add the Stilton and mustard. Season, then stir well, as the cheese may stick to the bottom.
Stir in the parsley just before serving. Carve the pork into thin slices. Serve with the sauce and green vegetables.
The pork en croute can be assembled up to 12 hours ahead, ready to roast. The sauce can be made up to one day ahead.
Recipes extracted from Mary Berry Simple Comforts by Mary Berry (BBC Books, £26)
Visit our Christmas recipes section for more festive ideas, including more from Mary Berry, or visit our pork recipes section