Bake pastry shell and set aside.
Get meringue ready to whip. But don’t make yet, it helps to have it as ready as possible so it can be applied to the hot lemon filling as soon as possible. The hot filling and fresh meringue seal better (with less likelihood of separation of the layers when done this way). Put the 3 egg whites in a grease-free stainless steel or glass bowl. Measure the cream of tartar sugar and have vanilla ready to add. This will all be mixed up after the filling is made.
Preheat oven to 325°.
For the lemon filling: combine sugar, cornstarch, and salt in a large sauce pan and set aside. In a medium size bowl combine the egg yolk, water, and lemon juice; beat with a whisk for about 2 minutes or until frothy, and then pour into the sugar mixture in the sauce pan.
Over medium heat cook lemon mixture, stirring constantly until it thickens and boils. Once it comes to a boil let it boil for 3 minutes, stirring constantly. Remove from the heat and stir in the butter and lemon zest. Pour into the baked pastry shell and smooth.
Now for the meringue: place the cream of tartar in with the egg whites and beat on medium speed with an electric hand mixer until soft peaks form (you can tell they are soft peaks when you lift the beaters and the mixture peaks and the tips curl). Once you’ve reached this stage, start slowing adding the sugar (about a tablespoon at a time) while beating on high speed. Once all sugar is added, continue beating on high speed until stiff peaks have formed (you can tell they are stiff by lifting the beaters up and noticing that the peaks stand tall and don’t droop or curl). Turn off mixer and add the vanilla and beat one more time until combine. Smooth the meringue over the hot lemon filling, taking care to seal it to the edge of the pastry. You can pile it billowy or smooth. I kind of like the look of little peaks and billows.
Bake at 325° for 25-28 minutes, until the meringue is a medium brown color. Let it cool completely and store the the fridge until ready to serve. You can add lemon rind curls for garnish if desired.