Preheat oven at 200°C.
Peel, core and slice the cooking apples. Add the raw apples, castor sugar and a small amount of butter in a saucepan and gently cook for 10 minutes. Add the blackberries into the cooked apple mixture, stir and set aside.
Sieve the flour into a large mixing bowl. Rub the butter and flour together, forming a breadcrumb-like texture. Add the brown sugar and oats into the mixture and stir well.
Spoon the fruit filling into an oven-proof dish and sprinkle the flour mixture over the top, ensuring the filling is covered.
Bake in the oven for 30-40 minutes until topping is golden brown.