Cream the butter and castor sugar until it is light and fluffy, beat in the oil.
Sift the dry ingredients.
Add the egg to the creamed mixture with 2 tablespoons of the flour mixture. Beat well. Then add the remaining dry ingredients and mix to form a smooth dough. Wrap the dough in plastic and refrigerate it for about 30 minutes.
Divide the dough into half. Lightly flatten a piece of dough and press it into the base and sides of the pie plate, ensuring and even thickness of about 3mm.
Combine the optional additional ingredients with the apple and spoon it into the unbaked crust.
Take the remaining ball of dough and using a coarse grater, grate it over the apples to form a light, crumbly topping. If the dough sticks to the grater, as it may do in a warm kitchen, just lightly dip the surface some flour.
Bake the tart in the centre of a pre-heated oven at 180°C for 25 minutes or until the surface is pale golden.
Serve warm with a dollop of whipped cream or vanilla or cinnamon ice cream.
The baked tart freezes very well. If you have a few tart plates or use disposable ones, you can make 2 or more at a time. Allow to cool and place in a plastic bag, draw out the air with a drinking straw and seal. Freeze for up to 6 months. Allow to defrost at room temperature and pop into a 180°C oven for 15-20 minutes to crisp the pastry and warm the filling.