Rub butter for pastry into the flour (or combine in a food processor until crumbly - much quicker!). Add the egg yolk and enough water to make ingredients just cling together - mix with a fork. Knead dough gently on a floured surface until smooth, roll out to the size of a 23 cm pie dish (if you wrap the pastry in plastic wrap and put it in the fridge for 30 minutes, it will be easier to handle). Cover the base and sides of the lightly greased pie dish with the pastry, trim, prick the base with a fork and cook at about 400° for about 12-15 minutes until lightly browned. Cool to room temperature.

While the pastry is cooking, prepare the filling. Combine sugar and cornflour in a saucepan, gradually stir in the juice and water, add the lemon rind and egg yolks and stir again. Add the butter and heat slowly, stirring constantly, until the mixture starts to bubble and is thick enough to 'mound' (should be quite thick and stiff). Remove from heat and cool to room temperature. Beat the egg whites until soft peaks form, gradually add sugar, beating between each addition, until sugar is dissolved and meringue is stiff.

Add lemon filling to pie crust, spreading evenly, top with meringue, drawing it into peaks with the back of a spoon. Bake at 350° for about 5-10 minutes, until meringue is lightly browned on the tips, but still soft.