Pre-heat the oven to 200°C.
Peel and core the apples and pears and cut them into cubes.
In a pan, melt 25g butter over a low heat and add the apples, pears and caster sugar and cook until soft. Add the blackberries and then transfer the fruit into an ovenproof dish.
In a bowl, add the flour, cinnamon and the remaining butter and using your hands rub it together to form a breadcrumb texture. Add the demerara sugar and repeat. Spoon the crumble mixture over the fruit and bake for 30 minutes.
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