2) Add the iced water and process until the dough comes together. Dump onto a well-floured board and form into a disk. Wrap in cling film and chill for at least 30 minutes. Meanwhile, preheat the oven to 190C/Gas 5.
3) Roll out the dough and fit into a 23cm tart dish with removable sides. Don't stretch the pastry when placing it in the dish or it will shrink during baking. Remove the excess pastry by rolling the pin across the top of the dish.
4) Line the pastry shell with a piece of buttered aluminium foil, butter side down, and fill with dried beans or rice. Bake for 10 minutes. Remove the beans and foil and prick the bottom of the shell all over with a fork to allow the steam to escape. Bake for another 15 to 20 minutes, until lightly browned. Set aside to cool.
5) Raise the oven temperature to 220C/Gas mark 7. For the lemon filling, cream the butter and sugar in the bowl of an electric mixer fitted with the paddle attachment for 1 minute. On low speed, add the eggs and egg yolks one at a time, then add the lemon zest, lemon juice and salt. It will look curdled, so don't worry.
6) Pour the lemon mixture into a small saucepan and cook over medium-low heat for 8 to 10 minutes until thick, stirring constantly with a wooden spoon. Whisk briskly when it starts to thicken and cook over low heat for 1 to 2 minutes, whisking constantly. Don't allow it to boil. It will be 78C on an instant-read thermometer. Pour into a bowl and cool to room temperature.
7) For the meringue, whip the egg whites, cream of tartar and 1/4 tsp of salt in the bowl of an electric mixer fitted with the whisk attachment on high speed until frothy. With the mixer still running, slowly add the sugar and beat until the meringue is thick and shiny, about 2 minutes.
8) Spread the lemon filling in the cooled pastry shell and pipe the meringue over it with a large star tip. Be sure the meringue covers the entire top and touches the edges of the shell, to prevent it from shrinking. Bake for 3 to 5 minutes, until the meringue is lightly browned. Cool to room temperature.