Pulse together flour, sugar, and salt in a food processor.
Add butter and lemon zest.
Pulse until coarse crumbs form. Add water gradually (approx. 1 tbsp at a time.) You might not need all of the water. Continue to pulse until crumbs become wet and begin to stick together, but do not over-process. It should not form a dough ball.
Dump crumby pie crust mixture onto a floured work surface and form a ball.
Shape into a disc. Cover with plastic wrap and refrigerate for at least 2 hours.
Preheat oven to 350°.
Roll out pie crust and fit into greased pie dish. Poke holes in the bottom. Ideally, use pie weights when you bake it. Bake until crust is golden. Set aside.
Beat egg whites and cream of tartar until they’re foamy.
Gradually add sugar, continuing to beat.
After all sugar is incorporated, continue to beat until the mixture is stiff and glossy. I used the third speed on my KitchenAid stand mixer for beating the meringue. The meringue should form a stiff peak when the beater is turned upside down.
Beat in vanilla.
Set aside egg yolks in a large heat-proof bowl.
Combine the sugar, cornstarch, and water in a saucepan over medium heat. Stir constantly until it boils (it should also be thickening.)
Let it boil for about a minute, continuing to stir so the sugar doesn’t burn. Definitely don’t skimp on the boiling or think it’s “close enough” – this is how you end up with runny filling! Depending on your range, you may need to turn the heat up above medium. I usually use a candy thermometer to make sure the temperature continues to ride toward boiling point and doesn’t plateau.
Pour about half of the hot mixture into the bowl with the egg yolks, stirring the yolks constantly.
Pour the egg mixture into the saucepan, continuing to stir.
Bring to a boil (tip from above about candy thermometer still applies), and boil for about 2 minutes while stirring constantly.
Remove the mixture from the heat.
Add the butter, lemon zest, and lemon juice. Stir until the butter is melted.
At this point, the mixture should be a fairly thick consistency.
Preheat oven to 375°.
Pour the filling into the baked pie crust.
Top with meringue. Make sure the meringue has a tight seal to the crust and doesn’t have any gaps. Ideally, you want the meringue to cover the crust edges so they don’t burn.
Bake at 375° for about 10 minutes on a middle oven rack, but watch the meringue so it doesn’t burn!
You can make the meringue at the same time as you're making the filling, but you can make it before the filling if you don’t want to multi-task.