Preheat the oven to 170C/325F/gas 3.
Grease the tart tin with butter.
To make the pastry sift the flour and salt into a large mixing bowl.
Cut the butter into very small chunks, add them to the bowl and stir them in.
Add the cold water and bring together to a wet dough with a lumpy consistency.
Cover with clingfilm and place in the fridge for 30 minutes.
Place the chilled dough on a floured work surface and shape into a rectangular block.
Roll the dough to a rectangle three times longer than it is short, keeping the edges straight and even. The rolled-out pastry should have a marbled effect from the lumps of butter.
Fold the top third down to the centre, then the bottom third up and over that.
Give it a quarter turn and roll out again to the original sized rectangle.
Repeat the folding, turning and rolling out 3 more times or until the marbling effect has disappeared. Fold once more and place in the fridge for 30 minutes.
Remove the dough from the fridge and place on a lightly floured work surface.
Roll the pastry out into a circle and line the tart tin with it. Prick the bottom of the pastry case with a fork.
Place in the fridge while you make the filling.
Place the chopped apple with a little water into the saucepan and simmer until the apple has softened.
Allow to cool a little and then place in the food processor and process to a purée.
Place the egg whites in a bowl and whisk to stiff peaks. Set aside.
Place the apple puree, butter, sugar, egg yolks and lemon zest in a large mixing bowl and beat with the electric whisk until thoroughly mixed.
Add a couple of large spoonsful of the whisked egg whites to the mixture and beat in. Then add the remaining egg white and fold in gently.
Pour the mixture in the pastry case and bake in the oven for 45 minutes.
After 30 minutes place a piece of foil over the ‘pudding’ to prevent the top burning.
Remove from the oven and sprinkle with caster sugar. Serve warm or at room temperature.
