Sift the flour, sugar and salt to combine in a bowl or using a stand mixer fitted with the paddle attachment. Cut in the butter by hand with a pastry cutter or on low speed until just small pieces of butter are visible and the mixture as a whole just begins to take on a pale yellow colour (indicating that the butter has been worked in sufficiently).
Stir the water and lemon juice together and add this to the dough all at once, mixing until the dough just comes together. Shape the dough into a disc, wrap and chill for at least 2 hours before rolling. Alternatively, the dough can be frozen for up to 3 months and thawed in the fridge before rolling.
Roll out the dough on a lightly floured work surface until it is in a circle that is just under ¼ inch thick. Lightly dust a 9” pie plate with flour. Press the dough into the pie plate and trim away any excess dough, pinch the edges to create a fluted pattern and chill for 30 minutes.
Preheat the oven to 400F. Line the chilled pie shell with tin foil and fill the foil with dried beans, raw rice or pie weights. Bake the pie shell for 20 minutes, then carefully remove the foil and weights and bake the crust for 8 to 10 minutes more, until the centre of the pie shell is dry-looking and just starts to brown a little. Immediately after removing the pie shell from the oven, brush the hot crust with a little of the whisked egg white. This will create a barrier to keep the crust crispy once filled. Reduce the oven temperature to 325F.
For the filling, whisk the sugar and cornstarch together in a medium saucepot, then whisk in the cold water. Have the other ingredients measured and nearby. Bring the sugar mixture up to a full simmer over medium-high heat, whisking as it cooks, until the mixture is thick and glossy.
Pour about a cup of this thickened filling into the egg yolks while whisking, then return this to the pot and whisk just one minute more. Whisk in the lemon juice and cook until the filling just returns to a simmer. Remove the pot from the heat and whisk in the butter then immediately pour the hot filling into the cooled pie shell (the filling will seem very fluid, but it will set up once chilled). Cover the surface of the filling with plastic wrap to keep it hot. Immediately prepare the meringue topping.
Whip the egg whites with the cream of tartar on medium speed until foamy, then increase the speed to high and gradually pour in the granulated sugar and icing sugar and continue whipping just until the whites hold a medium peak when the beaters are lifted.
Remove the plastic wrap from the hot lemon filling, then dollop half of the meringue directly onto the filling (the filling will still be very soft, so work gently). Be sure to spread the meringue so that it completely covers the lemon filling and connects with the outside crust, then use a bamboo skewer or paring knife to swirl the meringue just a touch (this will secure it to the lemon curd). Dollop the remaining meringue onto the pie and use the back of your spatula to lift up the meringue and creates spikes. Bake the pie for about 20 minutes at 325F, until the meringue is nicely browned. Cool the meringue completely to room temperature before chilling for at least 4 hours.
Tips for Lemon Meringue Pie Success:
- Adding the lemon juice at the end of cooking the curd filling ensures that it retains its fresh flavour, and also reduces the contact with the cooking cornstarch (its thickening power is reduced in the presence of an acid).
- It is critical that the filling is hot when spread the meringue over. If it cools, the meringue will sweat, creating a liquid layer in between the filling and itself.
- Be sure to spread the meringue so that it joins with the crust. This will also help prevent a moisture layer from forming, and will prevent the meringue from shrinking as it cools.
- A meringue that sweats or “beads” on top is a sign that the whites have been over whipped, overbaked or merely a sign of a humid day. When whipping, the whites should hold a medium peak when the beaters are lifted and should still appear glossy. Once baked, the meringue should be a light brown, with still a few white patches visible.