Preheat the oven to 190°C.
Fry the onion in butter and oil until softened.
Cook mushrooms and garlic with the onions until moisture evaporates then set aside.
Stuff the pork fillet with black pudding through a long incision.
Sear the pork fillet in oil for 4 minutes until browned.
Roll out the pastry to a 20cm x 30cm rectangle.
Layer ham and mushrooms on pastry then top with pork.
Seal the pork in pastry using egg wash and decorate with offcuts.
Bake the Wellington on a non-stick tray for 15 minutes.
Lower heat to 170°C, bake for 10 minutes, then rest for 10 minutes.
Fry sliced apples in the pork juices until golden.
Simmer mustard, stock, sage, and sherry until reduced by half then stir in cream.
Slice the Wellington and serve with the warm apple sauce.