Preheat the oven to 200c (400f, gas mark 6). You will need a 28cm (11”) loose bottomed, fluted flan tin.
To make the pastry, measure the flour and sugar into a bowl and rub in the butter until the mixture resembles fine breadcrumbs. Add the egg yolk and water and work to a firm dough. Wrap in cling film and leave to rest in the fridge for about 30 mins. On a lightly floured work surface, roll out the pastry and use to line the tin. Prick the pastry lightly all over with a fork, line with grease proof paper, fill with baking beans and bake blind in the pre heated oven for about 20 mins or until pastry is pale golden and dried out, removing the paper and beans for the last 5 mins.
Lower the oven heat to 150c (300f, gas mark 2). For the filling, finely grate the rinds from the oranges and squeeze out the juice. Put the rind, juice and cornflour into a small bowl and blend together using a teaspoon until smooth. Bring the water to the boil, and then stir into the cornflour mixture. Simmer gently, stirring until it forms a thick custard. Mix the egg yolks and sugar together in a bowl and stir in the custard. Bring back to the boil, whisking until just bubbling. Remove from the heat. Allow to cool a little and then pour into the pastry case. Spread the mixture out evenly.
For the meringue, whisk the egg whites in a large bowl until they form stiff peaks. Whisk in the caster sugar, a teaspoon at a time, still whisking on maximum speed until stiff and glossy. Spoon the meringue over the top of the orange filling and swirl into peaks. Bake for about 1 hour, until the meringue is crisp on the outside and soft underneath. Serve warm or cold.