Pre-heat your oven to 180 degrees and place a large pie dish or baking dish onto your bench.
Peel and core the granny smith apples and chop them into small pieces - approximately 2cm
Place the apple pieces into your pie dish and add the lemon juice, ½ cup of caster sugar and ½ teaspoon of cinnamon and mix until the apple is coated with all of the ingredients.
In a large bowl, add the melted butter, plain flour, rolled oats, ½ cup of caster sugar and 1 teaspoon of cinnamon and mix well to combine.
Using your hands, sprinkle the crumble mixture over the top of the apple pieces.
Place the Apple Crumble into your preheated oven and cook for 40 minutes or until golden on top and the apple has cooked through when tested with a knife.
Carefully remove the Apple Crumble from the oven and allow it to cool for 5 minutes before serving.