Make the pastry:
Combine the flour and powdered sugar in a bowl. Add the cold butter cubes and rub them into the flour with your fingers until the mixture looks like fine breadcrumbs. Pour in the beaten egg and mix gently until the dough starts to come together. If it feels too dry, add a teaspoon of cold water. Wrap the dough in plastic and chill it in the fridge for 20 minutes.
Preheat your oven to 350°F (175°C). Roll out the chilled dough on a lightly floured surface until it’s about ¼ inch thick. Carefully press it into a 9-inch (23cm) tart tin, trimming any excess dough from the edges. Prick the base a few times with a fork.
Bake the pastry case for 15 minutes until it’s just starting to turn golden. Remove it from the oven and set aside.
Prepare the lemon filling:
In a saucepan, mix the cornstarch and sugar. Slowly stir in the water, lemon juice, and zest. Place the pan over medium heat and cook, stirring constantly, until the mixture thickens and starts to bubble.
Whisk the egg yolks in a bowl. Gradually pour some of the hot lemon mixture into the yolks, whisking quickly to prevent them from cooking. Then pour the yolk mixture back into the pan with the rest of the lemon filling. Cook for another minute while stirring until smooth and thick. Remove from heat.
Pour the lemon filling into the warm pastry shell and smooth the top.
Make the meringue topping:
Using a clean, dry bowl, beat the egg whites with an electric mixer until soft peaks form. Slowly add the granulated sugar a little at a time while continuing to beat. Keep mixing until the meringue is glossy and holds stiff peaks.
Spread the meringue evenly over the lemon filling, making sure to seal the edges well so it sticks to the pastry.
Bake the pie at 325°F (160°C) for about 20 minutes or until the meringue is golden brown on top.
Let the pie cool for at least 30 minutes before slicing. This helps the filling set perfectly.
