Preheat oven to 200C (390F) fanbake, and line a slice tin with baking paper. I used an 18x27cm pan (approximately 7x10.5 inches).
If you're using frozen blackberries, measure them into a bowl then set aside to thaw a bit while you're making the crumble.
Mix all crumble ingredients, except for the coconut oil, in a large bowl.
Pour the melted coconut oil over the crumble ingredients, bit by bit, mixing well, until the mixture is wet and clumps together easily when squeezed. You may not need the whole quantity of coconut oil.
Measure a heaped cup of crumble mixture and set aside for the topping.
Press remaining crumble mixture into the slice tin, using the back of a spoon or a drinking glass to get it nice and even.
Sprinkle the corn flour over the blackberries and toss together. Tip the blackberries onto the crumble base and spread out evenly.
Evenly distribute the grated apple and sliced sage over the blackberries.
Use the reserved crumble mixture as a topping for the fruit, spreading out evenly but keeping it in rustic clumps. You're not aiming for perfection here. Pay attention to any gaps, and make sure everything is evenly distributed.
Bake for 30 minutes, or until a deep golden brown.
Remove from oven and allow to cool and set for 1-2 hours in the pan. Use the baking paper to gently lift the uncut bars onto a chopping board, before slicing into even portions. I sliced mine into 12 generous serves.