To make the pastry, place the flour, sugar, salt and Lurpak® butter in a large bowl and rub between your fingers until you have fine crumbs. Alternatively, put the ingredients into a food processor and whizz until you have fine crumbs. Add the egg yolk and water and gently bring the dough together. Make sure not to overwork the dough. Flatten the dough with the palm of your hand and wrap in cling film. Place in the fridge for a minimum of one hour.
Roll out the pastry and line a greased 20-22 cm tart tin. Leave the pastry hanging over the sides and place on a baking tray. Chill for a further 30 minutes. Heat the oven to 170c fan/ gas mark 4. Line the pastry tin with greaseproof paper and baking beans and place in the oven for 20 minutes. Remove the pastry tin from the oven and remove the beans and paper. Return to the oven for a further 5 minutes or until the pastry is cooked and golden. Remove from oven and using a vegetable peeler or sharp knife, shave off the excess pastry.
Meanwhile, put the lemon juice, zest and water into a saucepan and whisk in the cornflour until smooth. Stir in the sugar and egg yolks. Place the pan over a low heat and cook gently, stirring until the mixture is warm. Add the Lurpak® butter and bring to the boil, stirring continuously until the mixture is thick. Pour the lemon mixture into the cooled tart case and leave to cool completely.
Finally, make the meringue. In a large bowl whisk the whites until they are firm. Add the sugar, a spoonful at a time. Heat the oven back to 170C. To make the meringue, whisk the whites until soft peaks form. Add the sugar a spoonful at a time and continue to whisk until the meringue holds its shape and is shiny. Whisk in the cornflour. Spoon the meringue on top of the lemon meringue pie. Use a blow torch to brown the peaks a little and then bake for 10 minutes until the meringue is golden. Allow to cool before serving.