Preheat oven to 180°C (350°F). Grease a 23 cm (9 inch) pie dish.
Mix all the pastry ingredients in a medium size bowl or food processor. If the mixture is too crumbly, add some extra water and mix until it becomes a pliable dough. If the mixture is too wet, add a little more flour and mix again. Set aside ⅓ of the dough.
On a floured surface, roll out the pastry into a thin circle about 5 mm (¼ inch) thick. Transfer the pastry to your pie dish and press it against the base and sides to form a crust.
Trim off the excess pastry and crimp the edges if desired. Prick the bottom with a fork to let hot air escape when baking.
Toss all ingredients in a bowl until combined. Scoop the mixture into the pie dish and set aside.
Add the oats (and nuts if using) and additional sugar to the reserved pastry. Rub with your fingers to create a crumble. Sprinkle the mixture on top of your apples.
Bake the pie in the oven for 40-50 minutes or until the apples are tender and pastry is golden brown.
Serve with a few dollops of cream or ice cream. Enjoy!
* Apples were weighed after they were cored and still had their skin on. You're welcome to peel your apples if you prefer.