Add the flour and sugar from the "Crumble" ingredients into a bowl. Mix together.
Cut your butter into approx. 1cm cubes and add to the flour/sugar mix.
Wash your hands with reaaally cold water to make them as cold as posible!
Rub together the butter, sugar and flour with your fingertips until it forms small breadcrumbs.>Tip: when you think you're done, shake the bowl and if there are an big bits, they will come to the top - rub those in. Shake and repeat until all the big bits have gone.>If you've never got cold hands like me, you might find it easier to use an electric whisk. As soon as you start seeing the butter and flour joining into crumbs (like the pic), STOP! If you whisk beyond this point, it will start to form more of a dough. Tip the crumble mix over the top of the apples.
Gently push the mix down to form a more solid top. This will give you biscuit-y clumps of crumble.
If you're using oats, mix together the "Topping" ingredients and sprinkle over the crumble.