Put the butter and flour in a food processor and blitz until it resembles breadcrumbs, then add the sugar and pulse. Mix the egg yolks with 2 tsp cold water and pour into the blender with the motor running, to bring the pastry together.
Put the pastry onto a lightly floured surface and roll out to line a 23cm loose-bottomed tart tin. Chill for 30 mins, then trim the edges with a sharp knife.
Preheat your oven to 200C/180C fan. Take the pastry out of the fridge and prick the bottom of the pastry case with a fork, then line the pastry case with baking paper. Fill with baking beans and blind-bake for 15 mins. Remove the parchment and beans and bake for a further 10 mins or until the pastry is lightly golden and dry to the touch.
Lower oven to 180C/160C fan. While the case is baking, make the filling. Put the cranberries in a shallow pan over a medium heat with 2 tbsp water for 5 mins. Place in a blender and blitz until the fruit is pureed. Push the fruit through a sieve over a bowl; discarding what is left in the sieve.
Whisk together the cornflour, orange and lemon juices until smooth and add to a pan. Add the sugar and the cranberry purée, and heat until the mixture has bubbled and thickened, keep stirring constantly so that the mixture doesn’t catch! Remove from the heat, add the butter and beat until it has melted into the mixture.
Place back on the heat and add the egg yolks and whole egg to the pan, beating well. Keep stirring for 5 mins until the mixture thickens then remove from the heat and set aside.
Meanwhile, make the meringue. Put the egg whites in a large clean bowl and whisk to stiff peaks. Add the sugar, one large spoonful at a time, whisking between each addition until stiff again. Whisk in the cornflour.
Pour the filling into the pastry case, and top with the meringue mixture. Bake for 20 mins until the meringue is crisp and slightly coloured. Let the pie sit in the tin for 30 minutes, then remove and leave for at least another 45 minutes before slicing.