• Melt the butter in a non-stick frying pan over a medium heat. Add the onion and fry it for 3 to 5 minutes, stirring occasionally, until it’s soft and translucent.   Turn the heat right up, then add the calvados and let it bubble away until it has almost completely evaporated (about 1 minute).   Transfer the onion to a bowl to cool.
• Put the cooled onion in a mixing bowl with the apple, sage and black pudding. Season with salt and freshly ground black pepper and mix everything together well.
• Heat the olive oil in the frying pan used for the onion over a medium to high heat, add the pork and fry it, turning regularly (approx. every 45 seconds) until the meat is sealed and well browned on all sides. Remove it from the heat.   Preheat the oven to 200C/180C fan/gas 6.

To assemble and cook the wellington:

• Dust a work surface with flour and roll out the puff pastry into a rectangle slightly longer than the pork and wide enough to completely encase it (at least 35cm x 25cm).
• Lay slices of prosciutto over the pastry equal to the length of the piece of pork, making sure you leave a pastry border of at least 1cm all round them. Spread the black pudding mixture over the prosciutto (you may have some left over).   Pat the seared pork dry with kitchen roll, then place it along the centre of the prosciutto and black pudding mixture.
• Brush the edges of the pastry with beaten egg, then wrap the pastry around the pork to form a large sausage shape. Press the ends of the pastry together to make sure the parcel is completely sealed and tuck them underneath.  Use a fork to seal down the seams.  Place the Wellington on a non-stick baking tray, making sure it is seam side down.
• Decorate the top with offcuts of pastry or use a sharp knife to gently create a pattern on the pastry, if you like, then brush the whole thing with more beaten egg.
• Bake the parcel in the preheated oven for 25 minutes, then turn the temperature down to 180C/160C fan/gas 4 and cook for another 15 to 20 minutes, until the pastry is golden brown. Use a meat thermometer to check the internal temperature is at 63C. Remove the Wellington from the oven and leave it to rest for at least 10 minutes.   Cut into slices and serve with your choice of side dishes.