Combine the topping and crust ingredients. Press one half of the mixture into the bottom and sides of an ungreased 10-inch glass or ceramic pie plate, building up the sides to the top of the plate to form an edge. Reserve remaining mixture for the topping. To make the lime curd, combine the butter, lime juice, cornmeal, salt and 3/4 cup of the sugar in a large bowl over a pan of simmering water with the bottom not touching the water, and slowly cook, scraping the sides of the bowl and whisking occasionally, another 15 minutes, or until thickened. Cover with plastic and refrigerate. Stir the curd every 10 minutes or so to cool evenly.

Preheat the oven to 275 degrees. In a clean bowl, beat the egg whites with the cream of tartar and the remaining 1/2 cup sugar until stiff and glossy. Gently fold half of the beaten whites into the cool lime curd to lighten. Then fold in the remaining whites and the lime zest. (This should be very gentle; it is okay for a few streaks of white to be showing.) Pour the filling into the pie shell and sprinkle on reserved topping evenly to cover. Gently press the sides in so the top 1/2 inch of the crust remains bare. Bake 45 minutes, or until the pie has risen and coconut is golden brown. The filling should be slightly cracked and firm when pressed in the center. Cool completely, cover with plastic and refrigerate 6 hours to chill thoroughly. Serve cold.