Preheat the oven to 180C/350F/gas 4.
Grease the tart tin with butter.
To make the pastry sift the flour and salt into a large mixing bowl or food processor bowl.
Add the butter and rub in (or process if using food processor) until the mixture resembles fine breadcrumbs.
Place the egg yolk in a cup and add the ice cold water and whisk together.
Add to the pastry mix and bring together to make a dough adding more water if the dough is too dry. If using a processor add the egg mixture and whizz until the mixture forms a ball of dough.
Wrap the dough in clingfilm and place in the fridge for 30 minutes.
Place the pastry onto a lightly floured board and roll out to a 3mm thickness.
Line the prepared tart tin and prick the base with a fork.
Scrunch up a large piece of baking paper, open it out and place over the pastry. Fill with baking beans and bake in the preheated oven for 15 minutes.
Take out of the oven, remove the paper and baking beans and brush the pastry with the beaten egg. Return to the oven for a further 10 minutes. Remove from the oven and set aside to cool.
To make the filling place the egg yolks and condensed milk into a large mixing bowl and beat together with the electric hand whisk for 2-3 minutes.
Add the lemon zest and juice and beat in.
Pour the mixture into the pastry case, place in the oven and bake for 15-20 minutes until set.
Remove from the oven and set aside.
Reduce the temperature to 150C/300F/gas 2.
For the meringue topping, whisk the egg whites in a large mixing bowl to stiff peaks and then gradually whisk in the sugar until glossy and thick.
Pile the meringue onto the pie and spread-out with the palette knife, making peaks as you spread.
Place in the oven and bake for a further 10 minutes until the meringue has taken on colour. It should be fairly crisp on the outside and soft and marsh mallowy on the inside.
Leave to cool slightly before serving.
