Preheat the oven to 180°C/gas mark 6.
For the pastry, in a food processor mix together the flour, salt and sugar, then add the butter and process until the mixture resembles breadcrumbs. Cut the vanilla pod in half and scrap out the seeds, then beat together the whole egg and vanilla seeds and add to the pastry. Process until the pastry forms a ball. Wrap tightly in cling film and refrigerate for two hours.
Roll out the pastry on a lightly floured surface to 2mm thickness. Use to line a 24cmx4cm tart ring placed on a baking tray. Rest the lined tart ring in the fridge for 20 minutes. You will have left over pastry, wrap in clingfilm and freeze until another day.
Line the tart ring with greaseproof paper and fill with baking beans to keep the base’s shape and bake blind for about 20 minutes. Remove the greaseproof paper and baking beans and return to the oven for five-eight mins or until the pastry is starting to turn golden brown. Remove from the oven and brush with egg yolk and allow to cool.
While the pastry bakes start on the filling. Chop the rhubarb into 2cm pieces and place into a saucepan, cover with the apple juice bring to the boil. Skim and then cook on a medium heat for 15 minutes until soft and beginning to fall apart. Strain through a sieve and push the pulp through. Return to a clean saucepan and reduce till you have about 500ml of juice.
In a bowl whisk together the cornflour and sugar making sure there are no lumps left. Add to the rhubarb reduction and and over a medium heat cook stirring continuously until the mixture bubbles and thickens. Remove from the heat and add the butter and beat until mixed.
Separate the eggs keeping the whites to one side for the meringue. Add the whole egg to the yolks and beat until mixed. Return the pan to a medium heat and add the yolks and egg. Keep stirring and cook for four-five minutes until mixture thickens and just starts to bubble. Remove from the heat.
For the meringue: In a clean bowl whisk the whites to soft peaks, than add half the sugar tbsp at a time. Add the cornflour to the remaining sugar, mix and continue to add a tablespoon at a time until mixed and shiny.
Fill the pastry case with the rhubarb curd and level with a spatula or spoon then add the meringue starting from the sides and working inwards. Pile it high and give it a swirl then return the pie to the oven for 20 minutes until crisp and slightly browned. Check not to burn the top.
Leave to rest for about two hours before serving.