Prep the Apples: Peel, core, and quarter both types of apples. Toss gently in a large bowl.
Layer the Fruit: Mix demerara sugar and cinnamon. In your baking dish, layer half the apples, sprinkle one-third of the sugar mix, add blackberries, then the next third of sugar. Add remaining apples and top with the last of the sugar mixture.
Make the Crumble: In another bowl, combine flour, sugar, oats, and cinnamon. Rub in the cold butter until the mixture resembles breadcrumbs.
Assemble: Evenly scatter the crumble topping over the fruit—no need to press it down.
Bake: Preheat oven to 170°C (150°C fan, Gas Mark 3). Bake for 40 minutes until golden and bubbling. If it browns too fast, cover loosely with foil.
Serve Warm: Let cool for 5–10 mins before serving. Enjoy with custard, cream, or vanilla ice cream.