Preheat oven to 375 F°.
Peel apples and slice them using an apple slicer. I used this apple slicer from Amazon.
In a bowl, add apple slices and the rest of the filling ingredients. Gently mix with a spoon. The point is to get the slices seasoned but not to break the apple slices into smaller pieces —> you can also use your hands if easier.
In a separate bowl, mix the crumble ingredients using a spatula and gradually keep adding the flour. Once the mixture looks more like crumble, break it up using your hands.
Pour the filling into the crust.
Spread crumble topping over the filling until the filling is fully covered.
Cover with foil and place on a cookie sheet. Bake for 45 minutes then uncover and bake for another 5 minutes.
Allow it to cool for 15 minutes and then serve!