For the pastry, put the flour and salt in a bowl. Rub in the butter until it resembles coarse breadcrumbs. Gradually, add the beaten egg and combine to form a dough (keep any leftover egg for glazing the pastry).
Press into an oval shape, a little less than 1cm/½in deep. Cover with cling film and chill in the fridge for at least 30 minutes (if you are in a hurry, you can put it in the freezer for 10-15 minutes).
For the filling, put the bacon joint in a saucepan and cover with cold water. Bring to the boil and simmer for 45 minutes-1 hour, or until cooked. When ready, a skewer will go in and out easily.
Meanwhile, fry the onion and garlic in the butter until they are soft and golden-brown.
When the bacon joint is cooked, remove it from the cooking liquor and set aside to cool.
Preheat the oven to 180C/350F/Gas 4.
In a bowl, mix the cider, cream, mustard, brown sugar and chopped herbs together. Mix in the cornflour, using a whisk or fork to break up any lumps, and then season to taste with salt and freshly ground black pepper.
Once the meat is cool enough to be handled, cut it into 5mm/¼in thick slices
Grease a 2 litre/3½pint pie dish with butter. Layer half the sliced potatoes on the bottom of the dish, followed by half the cooked onions and garlic. Now add half the sliced apple and half the cooked bacon. Pour half the cream mixture over the layers.
Repeat the layers with the remaining ingredients.
Remove the pastry from the fridge. On a lightly floured work surface, roll the pastry out to a thickness of approximately 4mm/ ¼in. Place the pastry on top of the pie dish and trim any excess from the edges (keep any scraps for decorating).
Brush any leftover egg or a little milk over the pastry as a glaze.
Roll any scraps of pastry into shapes, such as leafs or apples, and use these to decorate the top of the pie. Make a hole in centre of the pie to allow steam to escape. Brush any decoration with egg glaze or milk. Use a fork to crimp the edges of the pastry.
Cook the pie in the oven for 45 minutes, or until the pastry is golden-brown and the filling is bubbling.