1 egg white, beaten
In a small bowl, beat together the egg, vanilla and icing sugar. Put the flour and salt in a food processor and combine. Add the butter and pulse until barely combined. Add the egg mix and pulse again for a few seconds. Transfer mix to a lightly floured surface and knead to bring together to form a ball. Flatten into a disc, wrap and chill in the fridge for at least an hour. When pastry is rested, cut two thirds from the disc and roll to fit a deep, 10-inch (25cm) tart tin. Chill for 15 minutes. Preheat oven to 180C. Blind bake pastry shell in tin using baking weights for 15 minutes.
For the filling, combine apples, sugar, butter and lemon zest in a large saucepan. Cook over a low heat until the apples soften, about 10 minutes. Set aside to cool.
Brush the partially cooked pastry shell with the egg white and fill with the cooled apples. Roll out the remaining pastry and, using a fluted pastry cutter, make 11-13, 1.5cm-wide strips to form a lattice pattern over the apples. Brush with egg white and bake for 30 minutes.