Lemon Meringue Tart

250g plain flour
¼ cup (35g) icing sugar
200g cold butter, chopped coarsely
1 teaspoon finely grated lemon zest
1 egg yolk
1-2 tablespoons iced water, as necessary
½ cup (75g) cornflour
1 ¼ cups (280g) caster sugar
3 eggs
3 egg yolks
4 lemons, zested and juiced
½ cup (125ml) water
100 g butter, chopped into 1 cm cubes
3 egg whites
1 cup (220g) caster sugar
½ cup (125ml) water
To make the pastry: process flour, icing sugar mixture, butter and zest until they resemble breadcrumbs. Add yolk and 1 tablespoon of the water; process again until pastry begins to come together into a ball. If necessary, gradually add more water, a teaspoon at a time, until pastry does form a ball. Tip onto a lightly floured work surface and knead briefly until smooth. Shape into a flat disc. Wrap in plastic wrap and refrigerate 30 minutes.
Preheat oven to 220°C or 200°C fan-forced. Roll out pastry between 2 sheets baking paper until 3mm thick. Press pastry into 24cm loose bottomed flan tin. Trim edges and dock base (to dock means to prick base using a fork). Line with baking paper and fill with dried beans, lentils or baking beads. Bake in preheated oven 15 minutes or until lightly browned. Remove paper and beans; return to oven. Bake a further 5 minutes or until pastry is lightly golden brown all over base. Allow to cool.
For lemon filling: Combine cornflour and sugar in medium bowl. Gradually add combined eggs, yolks, zest, juice and water; whisking to ensure mixture is lump free. Microwave on medium (50%-60%) for 10 to 15 minutes, stirring every minute or until mixture is thick. Or place mixture in medium bowl over a saucepan of simmering water. Stir occasionally until thickened – about 30 minutes. Beat butter into lemon mixture. Cover closely with plastic wrap and cool.
For meringue: Place egg whites in large bowl of electric mixer. Combine sugar and water in medium heavy based saucepan. Stir over low heat, without boiling, to dissolve sugar. Brush down sides of pan with wet pastry brush to remove any sugar crystals. Simmer, uncovered, without stirring, until mixture reaches firm ball stage (119 – 121°C) on a sugar thermometer (a teaspoon of mixture will form a firm ball when dropped in a cup of cold water).
Just before sugar reaches temperature, begin to beat egg whites until stiff peaks form. With motor operating, gradually pour sugar mixture onto egg whites until fully incorporated.
Spoon lemon mixture into pre-baked pastry case. Level top with a small palette knife. Pile meringue mixture on top, ensuring lemon mixture is completely covered. Return pie to preheated oven (220°C or 200°C fan-forced) and bake 10-15 minutes or until top is golden brown and crisp. Stand 5 minutes before serving.