Preheat oven to 180°c (360°f) or 170°c (340°f) if fan assisted.
In a mixing bowl, add the peeled and chopped apples, caster sugar and cinnamon. Toss to coat the apples. Transfer to a small, deep baking dish and leave to one side as you prepare the crumble.
In a mixing bowl, add the flour, brown sugar and cinnamon. Mix together until combined.
Add in the melted butter and mix again until it resembles chunky breadcrumbs.
Add the oats and chopped nuts and mix together.
Sprinkle the crumble topping all over the apples.
Bake for 30-45 minutes or until the apples are soft. To check if it's done, use a sharp knife to pierce into the apples at the bottom, if they are soft and give way easily, it is ready.
Allow to cool slightly but serve warm.
Cooled leftovers can be transferred to an airtight container and placed into the fridge for up to 3 days.
Leftovers can be reheated in the microwave for 20-30 seconds before serving.
