To make the filling, peel apples, core and cut into thin wedges. Put apples, lemon rind, caster sugar, and water in the saucepan over low heat. Cover and simmer for 10 minutes or until apple is soft. Add the blueberries and simmer for another 1 minute. Set aside to cool.
Preheat oven to 180 degrees C. Roll out the large pastry (or combine 2 pastries together) on a lightly floured surface into a 24cm disc. Line a round 20cm springform pan with the pastry disc to cover the base and about 2 cm up on the side.
Strain filling to remove excess moisture and transfer to the pan. Roll out another pastry and cut into 2cm wide strips. Arrange the strips in a lattice pattern over the filling and press to seal. Sprinkle with demerara sugar on top.
Bake for 30 minutes. Set aside to cool for 10 minutes.
Serve with cream or custard and vanilla ice cream.