Pastry: Sift flour and salt into a bowl. Add the butter then rub in until the mixture resembles fine breadcrumbs. Add enough water to form a firm dough. Knead lightly until smooth and free from cracks.
Roll out pastry on a lightly floured work surface to a round large enough to line the base and sides of a 20cm springform tin.
Line the pastry shell with baking paper and cover with a layer of dried beans to prevent the base blistering. Place in a preheated oven at 200 degrees C for 10-15 minutes until the sides of the flan are set and golden.
Remove the baking paper and beans (which can be reused) and return the pastry shell to the oven for about 5 minutes or until the base is crisp. Allow the pastry case to cool.
Filling: Put the apples in a saucepan with the caster sugar, butter, lemon zest and water. Cook gently until the apples are tender then beat to a smooth puree.
Cool the mixture slightly then beat in the egg yolks. Put the mixture into the pastry shell. Place in the oven and bake at 180 degrees C for 20 minutes.
Meringue: Whisk the egg whites until stiff then whisk in the remaining caster sugar.
Spread the meringue over the top of the apples. Return to the oven for 15-20 minutes until golden. Serve hot or cold with cream.