Viennese Apple Pie

Preparation time: 20 minutes
Cooking time: 30-35 minutes
Serves: 8-10
50g/2oz sultanas
3 tbsp dark rum
225g/8oz plain flour
50g/2oz ground almonds
175g/6oz butter
150g/5oz caster sugar
grated zest and juice 1 lemon
1 egg, separated
675g/1½ lb Bramley apples
25g/1oz walnuts, chopped
50g/2oz sponge fingers, crumbled
1 tsp ground cinnamon
a little granulated sugar to sprinkle on top