Pastry: Cream the butter and sugar in a mixer and add the eggs one at a time. Sieve in the flour and bring together into dough. Wrap in cling film and chill for 1 hour before using. Roll out the pastry on a floured surface and push into an 8 inch tart tin. Place some parchment on the surface of the pastry and fill with baking beans. Bake blind for 20 minutes before removing the parchment and baking for a further 15 minutes.
Lemon curd: Place the lemon juice and zest into a small saucepan and bring to a boil. Whisk the eggs and sugar together and add to the lemon juice. Put back on the stove on medium heat and cook, stirring, until thickened. Remove from the heat and whisk in the unsalted butter until it has fully melted. Pour into a bowl, place cling film right onto the surface of the curd so it does not form a skin, and chill in the fridge until ready to use.
Meringue: Place the sugar into a small, heavy saucepan and cover with the water. Bring to a boil until it reaches 112 degrees C using a sugar thermometer. Whisk the egg whites in a clean bowl until they peak and slowly add the sugar syrup. Whisk until it cools down.
To assemble: Fill the tart shell with the chilled lemon curd and smooth off the surface using a flat knife. Top with the meringue, then place under a hot grill and brown lightly. Alternatively, use a pastry blow torch.