Preheat your oven to 325ºF.
For crust: in a foodprocessor, add the flours, arrowroot starch, granulated sugar, xanthan gum and saltand pulse until well combined.
Add the butter pieces andpulse until a coarse crumb-like mixture is formed.
While the motor isrunning, slowly add ice water and pulse on low speed until a dough forms.
Arrange a large piece ofparchment paper onto a smooth surface and dust with some almond flourgenerously.
Place the dough onto theparchment paper and with your hands, pat into a circle.
Cover the dough circlewith another large piece of parchment and with a rolling pin, roll into an11-inch circle.
Arrange the dough circleinto a 9-inch pie plate and gently press into the bottom and up the sides.
With a fork crimp theedges of the crust and then prick all over.
Bake for approximately 12minutes.
Remove from the oven andset aside to cool completely.
Now, Preheat your oven to300 ºF.
For filling: in a bowl,add the egg yolks and beat until smooth.
Slowly add ½ cup of thewater and beat until well combined.
In a pan, add 1 cup ofthe water, sugar, salt and lemonzest and salt over medium heat and bring to a boil, beating frequently.
Slowly, add the egg yolksmixture into the pan, beating continuously.
Reduce the heat to lowand cook for about 1 minute, stirring continuously.
Remove from the heat andstir in the lemon juice and butter until smooth.
Sprinkle the xanthan gumon top and with a wire whisk, beat vigorously until well combined.
Meanwhile, in a smallbowl, dissolve the gelatin into the remaining 2 tablespoons of water. Set aside for about 2 minutes.
Add the gelatin mixtureinto hot lemon mixture and beat until well combined.
Cover the pan and setaside.
For topping: in a largebowl, add the egg whites, cream of tartar and salt and beat until frothy.
Slowly add the powdered sugar, granulated sugar and vanilla extract and beat until stiffpeaks form.
Place the warm fillingover the crust evenly.
Dollop the meringue overthe filling evenly and with the back of a spoon, swirl it slightly.
Bake for approximately 20minutes or until top is golden.
Remove from the oven andset aside to cool for about 20 minutes.
Refrigerate for at least3 hours before serving.