Preheat oven to 170°C and line a 9 x 9 inch baking tray with baking paper.
Add the packet crumble mix and very soft butter to a bowl. Rub together until it forms a crumble that holds together when pressed but still breaks apart easily.
Press most of the crumble mixture firmly into the base of the lined tray, reserving some for the topping.
Drain the peaches very well, then add them to a bowl with the cinnamon and cornflour. Mix together until the peaches are evenly coated.
Spread the peach mixture over the crumble base.
Add the oats to the remaining crumble mixture and mix together until combined. Sprinkle the crumble topping evenly over the peaches.
Bake for 40–45 minutes or until golden.
Allow the slice to cool completely before slicing.
