Lightly butter a 10-inch tart pan.

TO MAKE THE CRUST, combine the nuts, flour, and sugar in a food processor. Add the butter and process until just combined. Add the egg, vanilla, and lemon zest. Process until all the ingredients are well mixed.

Press half the mixture into the prepared pan. (Freeze the extra dough, well wrapped in plastic.) Chill for at least 30 minutes. Preheat the oven to 350°F. Bake the crust for 20 to 25 minutes, or until the shell is golden brown. Let cool on a rack before filling.

TO MAKE THE FILLING, in the top of a double boiler whisk together the flour, sugar, egg yolks, water, and lemon zest. Heat the mixture, stirring constantly, over simmering, not boiling, water until thick.

Remove the mixture from the heat and whisk in the lemon juice. Allow the filling to cool for 15 minutes, then transfer it to the cooled macadamia nut crust.

Preheat the oven to 300°F.

TO MAKE THE MERINGUE, in a large bowl beat the egg whites with a balloon whisk or electric mixer until frothy. Add the cream of tartar. Continue beating the eggs until they hold soft peaks, then gradually add the sugar, 1/4 cup at a time. Beat until the mixture is stiff and glossy.

Heap the meringue on top of the, filled tart, and then sprinkle the top of the tart with the reserved tbsp of sugar.

Bake for 30 minutes or until the meringue is a light brown color. Allow the tart to cool before serving.