Sift the flour, nutmeg and salt into a large bowl.
Cut the butter into small cubes and rub into the flour with your finger tips until the mixture resembles fine breadcrumbs.
Stir in the oats and sugar and rub in a little more until the mixture begins to clump together.
Chill while making the filling.
Pre-heat the oven to 190C/375F/G5.
Peel and core the apples and cut into chunks and place in a bowl with the blackberries.
Combine the sugar, cinnamon and cornflour.
Sprinkle over the fruit and toss to coat.
Place the fruit in a shallow ovenproof dish, leaving 2.5cm in space for the topping.
Scatter the crumble mixture over the fruit and sprinkle with the demerara sugar.
Place on a baking sheet in the top third of the oven and bake for 40 minutes or until the crumble is golden brown and the fruit is soft when tested with a skewer.
Serve with custard, whipped or clotted cream.