To make pastry, sift flour and salt into a bowl. Add the fat, cut into small pieces, and 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 (8in) loose-based fluted flan tin.
Bake blind, line the pastry case with greaseproof paper and cover with a later of dried beans to prevent the base blistering. Place in a preheated oven, 200C (Gas 6), for 10-15 minutes until the sides of the flan are set and golden. Remove the lining paper and beans (which can be reused) and return the flan to the oven for about 5 minutes or until the base is crisp. Allow the pastry case to cool.
Put the apples in a saucepan with 50g of the caster sugar, butter, lemon rind 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 case. Place in the oven and bake at 180C (Gas 4), for 20 minutes.
Whisk the egg whites until stiff, whisk in the remaining caster sugar, then spread the meringue over the top of the apples. Return to the oven for 15-20 minutes until golden. Serve hot or cold with pouring cream.