To make the pie filling, wash, peel and grate the apples.
Add them to a pan together with the cinnamon and light brown sugar.
Give everything a good mix, and cook the apples on a medium heat until all the apple juice released has been absorbed.
Remove the pan from the heat and set aside to cool.
To make the crumble, sift the flour in a large mixing bowl, add the sugar and oats, and mix well.
Pour over the melted butter, and use a spatula to combine everything into a sticky crumbly mixture.
Line a square baking tray with non-stick parchment paper ( my tray is 20 cm /8 iches in diameter).
Transfer half of the mixture to the pan, and use a spatula or spoon to press it down well to have a firm base.
Top it with the apple filling, then scatter the remaining crumble mixture over the apples.
Bake in the preheated oven at 180 degrees Celsius ( 350 Fahrenheit) for 50 minutes until golden.
Remove the tray from the oven, allow it to cool slightly, then slice the crumble into squares.
Allow the squares to cool down completely before transfering them from the tray.