Combine sugar, cornstarch, and water in a saucepan. Cook on medium low till boiling, about 7 minutes.
Whisk yolks and sugar in separate bowl, temper yolks by adding ¾ of the boiling mixture to the yolks and sugar. Whisk together and slowly add back into saucepan liquid while stirring.
Add butter, salt, lemon juice and zest, cook about 3 more minutes. Should be bubbling and thickened. Take off heat.
In a mixer add egg whites and cream of tartar, beat on high until foamy. Slowly start to add sugar. Beat till stiff peaks.
Pour lemon pudding into prepared pie crust.
Add spoonfuls of meringue over hot lemon pudding. Using a spatula move the meringue around to the crust. Completely cover the top, all the way to the edges.
Bake 25 - 30 minutes. Till meringue is lightly browned.