Mix sugar, sea salt, and cornstarch. Add the boiling water and grated lemon rind. Cook over medium heat, stirring constantly until thickened. Simmer 10 minutes. Stir in the butter.
Beat the egg YOLKS with the lemon juice. Gradually stir the egg yolk mixture into the hot filling and mix until well blended. Strain the mixture into the baked pastry shell. Bake 10 minutes at 400 degrees F.
Make the meringue: In a medium bowl, beat the three egg WHITES. Add the lemon juice and a tiny pinch of salt. Begin beating slowly at first, which will produce small, stable bubbles. After a minute or so, increase the speed and beat until soft peaks form. Beat in 6 tablespoons of superfine sugar, 1 or 2 tablespoons at a time until stiff peaks form, which will several minutes. If you don't have superfine sugar, you can make your own by grinding granulated sugar in a food processor until very fine. This will make about 4 cups of meringue.
Spread the meringue onto the pie being sure to "seal" the edges by having the meringue touch all the edges of the pastry shell. Bake 20 minutes more at 375 degrees F.
There are two important things to know about making great meringue. 1) Be certain your bowl is absolutely clean. Don't use a plastic bowl or an aluminum bowl because both can retain tiny amounts of oils from food even after being washed. Copper works best, but stainless steel or glass works well too. 2) The egg whites should be at room temperature.