Put the apple chunks and blackberries into an ovenproof dish. Sprinkle the sugar and cinnamon over and gently mix together to avoid blackberries breaking up.
Put the flour, butter, porridge oats, sugar and cinnamon into a bowl. Rub together, then squash and break to give your crumble some texture once cooked.
Sprinkle the crumble mixture over the fruit, and bake in the oven for around 30 minutes until the crumble is golden and the fruit is bubbling around the sides.