Preheat oven to 180°C/ gas 5
Sieve flour, sugar and salt into bowl. Rub in the butter until it resembles breadcrumbs. Mix in egg yolk and enough water to make a stiff dough. Knead until the pastry is smooth and transfer to the fridge to chill for an hour. Grease tin and roll out pastry into a circle 3cm larger than the tin. Line tin evenly with pastry, moulding it into the corners and up to the sides to exclude air, taking care not to stretch the pastry. Trim excess pastry, mould up to the top of the pastry to form a small ridge above the rim of the flan tin. Prick all over with a fork and chill again if you have time.
Place a circle of parchment paper on top of pastry and cover with baking beans and bake blind for 15 minutes. Reduce oven to 130°C/ gas, put pastry back into oven for a further 5 mminutes to dry out.
Wash, zest and juice lemons. Whisk cornflour, lemon zest, 5 tablespoons lemon juice and water in saucepan. Bring slowly to the boil, stirring all the time. Simmer until the mixture begins to thicken and bubble. Remove from the heat and stir in the sugar. Cool and then whisk in egg yolks. Pour the mixture on top of the pastry.
Whisk egg whites in a clean bowl until stiff peaks begin to form. Whisk in half the sugar until stiff again. Fold in the remaining sugar. Pile onto lemon mixture using a fork to form peaks. Bake in a cool oven at 140°C/ gas 2 for about 25 minutes until the meringue has crisped and the tips have turned golden brown.
Serve hot or cold.