Prepare your pie crust or use a store bought pie crust. Preheat the oven to 200C/400F.
In a large mixing bowl, prepare your apple pie filling. Add your sliced apples, sugar of choice, cinnamon, and flour and mix well.
In a separate bowl, prepare your crumb topping. Add your sugar of choice, flour, and chopped walnuts, and mix well. Add your melted butter and mix until a crumbly texture remains.
Transfer the apple pie filling into the prepared pie crust. Cover the top of the pie with the crumble mixture.
Bake the pie for 45-50 minutes, or until the crumble has turned golden brown. Remove from the oven and let sit for 10 minutes, before serving.
TO STORE: Leftover apple pie should be stored in the refrigerator, completely covered. The pie will keep well for up to 2 weeks. Let the pie sit at room temperature for 30 minutes, before enjoying it. If you'd like it warm, heat it up in the microwave for 30 seconds. TO FREEZE: You can either freeze the pie as a whole or as individual slices. If you freeze the whole pie, cover in plastic wrap completely. If you do individual slices, place them in a shallow container. The frozen pie will keep well for up to 6 months. Thaw overnight in the refrigerator. TO REHEAT: Your pie must be room temperature before reheating. Preheat the oven to 180C/350F and once warm, heat for 10 minutes.