Preheat the oven to 200°C.
Rub flour, icing sugar, and butter together in a bowl until the mixture looks like breadcrumbs.
Add the egg yolk and 30ml of water to form a firm dough.
Wrap the dough in cling film and chill for 30 minutes.
Roll out the pastry and line a 25cm loose-bottomed flan tin.
Bake the pastry blind for 20 minutes before lowering the oven to 150°C.
Blend orange rind, juice, and cornflour in a small bowl.
Add 600ml of boiling water to the orange mixture and simmer until it becomes a thick custard.
Stir egg yolks and sugar into the custard and boil briefly while whisking.
Pour the cooling custard into the pastry case and spread evenly.
Whisk egg whites to stiff peaks and gradually add caster sugar at high speed.
Spoon meringue over the filling and swirl into decorative peaks.
Bake for 60 minutes until the exterior is crisp.