Heat butter and oil in a large pan and sauté the onions until just transparent.
Add the apples to the pan with the chopped sage and continue cooking, stirring all the while, for a further 5-8 minutes. Remove from the pan with a slotted spoon and then add the pork to the pan and brown quickly on all sides.
Stir in the flour and then add the cider, bring to the boil, stirring continuously.
Return the apple mixture to the pan along with the mustard and simmer, covered for 20 minutes until pork is tender Allow to cool.
Heat oven to 200ºC (180ºC fan assisted oven)/400ºF/Gas 6.
Place cooled filling in 4 individual pie dishes. Roll out pastry and use to top pie dish – see shortcrust pastry pie lids step by step.
Bake for approx. 20 minutes until pastry is golden brown. If necessary add a little more cider or water at point 4 to give a good, moist sauce to fill.