Preheat the oven to 350°F (175°C)
Prepare and bake a pie crust in a 9-inch pie dish
Separate eggs, reserving the yolks and whites
In a saucepan, whisk together sugar, cornstarch, salt, water, lemon juice, and lemon zest
Cook over medium heat until thickened and bubbling
Temper the egg yolks with some of the hot mixture
Return the yolk mixture to the saucepan and cook briefly
Remove from heat and stir in butter
Pour the lemon filling into the baked pie crust
Beat the egg whites with cream of tartar until soft peaks form
Gradually add sugar and beat until stiff, glossy peaks form
Spread the meringue over the hot lemon filling, sealing to the crust edges
Bake until the meringue is golden brown
Cool completely before slicing