Add the flour, sugar and butter to a large bowl and rub in with your fingertips or an electric mixer until you have fine crumbs. Measure out 250 g (8 oz), add the flaked almonds and reserve this for the streusel topping.
Mix in just enough water to the remaining pastry crumbs to form a soft but not sticky dough. Knead lightly until smooth, then roll out on a floured surface until a little larger than a buttered 28 cm (11 inch) fluted loose-bottomed tart tin. Lift the pastry over a rolling pin, drape into the tin, then press over the base and sides. Trim off the excess pastry with scissors so that it stands a little above the top of the tin. Chill for 15 minutes.
Make the filling. Mix the sugar and cornflour together. Quarter, core and peel the apples, slice and add to the sugar mix with the blackberries. Toss together gently, then pile into the tart tin. Spoon the streusel mix on top and stand the tin on a baking sheet.
Bake in a preheated oven, 190°C (375°F), Gas Mark 5, for 40–45 minutes, checking after 25 minutes and covering with foil if the almonds and edge of the pastry seem to be browning too quickly. Leave to cool in the tin for 15 minutes, then dust the top with icing sugar. Remove the pie from the tin and cut into wedges. Serve with spoonfuls of clotted cream.