Heat the oven to 350 degrees. Put graham crackers in food processor and whirl until you have fine crumbs. Add oatmeal, brown sugar and salt, and then pulse to incorporate. Add melted butter and pulse to combine. Press crumb mixture into 9″ pie pan, covering the bottom and sides of the pan. Bake for 12 minutes. Remove from the oven and cool.
In a stainless steel or copper bowl whip the egg whites until they are foamy. Add cream of tartar and continue whipping until the egg whites form soft peaks. Combine superfine sugar and 1 T cornstarch in a separate small bowl. Add the sugar and cornstarch to the egg whites a few tablespoons at a time, whipping until stiff peaks form. Set aside while you prepare filling.
In a medium saucepan, whisk together egg yolks, water, milk, sugar, lemon juice, lemon zest, 5 1/2 T cornstarch and salt. Whisk over medium heat until the mixture begins to boil. Reduce heat to low and continue to whisk while the mixture thickens, about 3-4 minutes. You know the mixture is done when it is thick and heavily coats a wooden spoon. Remove from heat and whisk in butter. Immediately fill the crust with the filling while the filling is still hot.
Give the meringue one last whip to freshen it and spoon meringue onto the filling, covering the filling completely, so that it touches the crust all the way around. I like to mound more meringue in the middle of the pie and slope it down to the edges of the pie. Use the back of a spoon to swirl the meringue into little peaks all over the top of the pie. Bake 10 – 12 minutes until the meringue has nicely browned. Let cool and refrigerate three hours before serving.