Set aside to cool. For the crumble topping, add the flour to a mixing bowl and rub in the butter until the mixture resembles fine breadcrumbs and no large lumps of butter are left. Stir in the ...
Rhubarb, apple and blueberry: Reduce the rhubarb to 300g/10½oz and add 125g/4½oz fresh or frozen blueberries to the fruit mixture, just before adding the crumble topping.