Add the flour, butter and sugar to a food processor. Process 1 min/speed 7.
Add the egg and vanilla essence mix to combine 15sec/speed 7
Place the base mixture into the baking tray and press the dough down to form an even layer.
Refrigerate the base.
Apple Filling
Peel and core the apples. Roughly dice the apple into 1 1/2cm cubes.
Add the apples lemon zest, cinnamon and apple sauce to a large bowl. Stir to combine.
Reserve for later use.
Crumble Layer
In the food processor add flour, butter and sugar. Process until the mixture looks like breadcrumbs. 1min/speed 7
Assembly
Spread the apple mixture over the base of the slice.
Using fingers add the crumble in clumps to the top of the apple layer. You want to squeeze bits together so the mixture isn't too small and crumbly on the top.
Bake for 45 minutes until the crumbs are lightly golden.
The slice will be soft whilst its hot but once refrigerated it will become much easier to cut neatly.