In a bowl, mix the sugar, flour, cornstarch, and salt together.
In a saucepan over high heat, boil 2 cups water; add the sugar mixture and cook, stirring constantly, until mixture just begins to thicken.
Reduce heat to low and cook until thickened; whisk in the beaten egg yolks and cook 2 minutes longer on low heat.
Remove from heat, stir in the lemon juice and rind and combine well; let cool for 5 minutes, then pour into previously-baked pie shell. If you like, you can cover with meringue (use "Weepless Meringue #20159" recipe).