Lemon merinque pie

Ingredients
Pastry:
200 g plain flour, plus extra for dusting
100 g butter, diced, plus extra for greasing
50 g icing sugar, sifted
finely grated rind of 1 lemon
1 egg yolk, beaten
3 tbsp milk

Filling
3 tbsp cornflour
300 ml cold water
juice and grated rind of 2 lemons
175 g caster sugar
2 eggs, separated

To make pastry, sift the flour into a bowl and rub in the butter. Mix in the remaining ingredients. Knead briefly on a lightly floured surface, then leave to rest for 30 minutes.
Preheat the oven to 180C/300F/gas mark 4. Grease a 20-cm ovenproof pie dish. Roll out the pasrty to a thickness of 5 mm and use it to line the dish. Prick with a fork, line with baking paper and fill with baking beans. Bake for 15 minutes. Remove from the oven. Reduce the oven temperature to 150C/300F/gas mark 2.
To make the filling, mix the cournflour with little water. Put the remaining water into a saucepan. Stir in the lemon juice and rind and cournflour paste. Bring to boil, stirring. Cook for 2 minutes. Cool a little. Stir in 5 tablespoons of the sugar and the egg yolks, then pour the mixture into the pastry case. In a bowl, whisk the egg whites until stiff. Gradually whisk in the remaining sugar and spread over the pie. Bake for 40 minutes. Remove from the oven and serve.