LEMON FILLING:
In a small bowl, whisk the egg yolks and set aside. Save the egg whites for the meringue.
In another small bowl whisk the water, gelatin, and xanthan gum. Set aside.
In a large saucepan over medium heat, combine the lemon juice, lemon zest, Powdered Sugar, salt, and whisked egg yolks. Bring the mixture to a simmer.
Slowly whisk in the gelatin mixture. Whisk constantly for 2-3 minutes over medium heat.
Add in the butter and stir until melted.
Remove from heat and pour the lemon filling into the cooled pie crust. Place the pie in the fridge overnight or for at least 5-6 hours.
KETO MERINGUE:
Preheat the oven to Broil.
Using a hand or stand mixer, beat the egg whites until stiff peaks form. Add Powdered sugar and continue mixing on high speed until the meringue is glossy and smooth.
Place small dollops of the meringue on top of the lemon filling. Smooth over the top, making swirls with the back of a spoon.
Place the keto lemon meringue pie under the broiler for 2-3 minutes, until the meringue is golden brown. Alternately, to achieve perfectly browned edges, you can use a blow torch.
Keep refrigerated until ready to serve. Serve cold.