Put the rack in the middle, preheat the oven to 160°C (325°F).
Lemon Filling: In a saucepan, combine the sugar, cornstarch, salt, lemon zest, and water. Bring to a boil over medium heat, stirring constantly until it thickens (the preparation must reach a boil to allow the starch to thickens). Reduce the heat and cook for 1 minute. Remove from the heat and set aside 75 ml (1/3 cup) of this mixture for the meringue.
In a bowl, combine the egg yolks, butter and lemon juice with a whisk. Whisk in a ladle of the hot mixture, and then pour back into the saucepan and stir (if the yolks were added directly to the hot mixture, they could coagulate and form lumps). Return to the heat and bring back to a boil, stirring constantly. Reduce the heat and cook for 1 minute to cook the yolks. Set aside. ?.
French Meringue: In a bowl, beat the egg whites with the cream of tartar with an electric mixer until soft peaks form. Gradually add the sugar, beating until stiff peaks form. Whisk in the reserved curd mixture (adding a little starch to the meringue prevents it from forming water droplets on its surface as it cools).
Reheat the filling and pour into the pie shell. Spoon the meringue over the filling (a hot filling cooks the underside of the meringue, which prevents seeping from the pie). Spread evenly, taking care to touch the edge of the crust. Form decorative peaks. Bake for about 20 minutes or until the meringue is golden brown.
Let cool completely at room temperature, about 5 hours. Preferably enjoyed the same day.