Set aside 2 teaspoons of the flour, then put the remaining flour and the butter in a large bowl. Rub the butter into the flour using your fingertips until the mixture resembles fine breadcrumbs. Stir through 1 tablespoon of the sugar, then add 1-2 tablespoons cold water.
Mix it in with your hands, squeezing all the crumbs together with your fingers – the dough should be soft not sticky. Turn the dough out on to a sheet of baking parchment and shape it into a ball. Wrap the dough in cling film and chill in the freezer for 15 minutes. Meanwhile, preheat the oven to 180°C, fan 160°C, gas 4.
In a bowl, toss together the apples, berry mix, and the remaining flour and sugar. Lightly grease a 1-litre pie dish and tip in the apple mixture. Remove the pastry from the freezer and coarsely grate the dough over the top of the pie filling.
Bake for 30-35 minutes until the pastry is golden. Serve with the warmed custard.