Preheat your oven to 350°F (175°C).
For the crust: In a mixing bowl, combine almond flour, erythritol, melted butter, egg, vanilla extract, and salt. Mix until a dough forms. Press it into a 9-inch pie dish and prick the bottom with a fork. Bake for 12-15 minutes, until lightly golden. Let cool.
For the filling: In a saucepan over medium heat, whisk together egg yolks, erythritol, lemon juice, lemon zest, and butter. Stir until thickened (about 5-7 minutes). If using gelatin, add it and whisk until dissolved. Remove from heat.
Pour the lemon filling into the cooled crust and set aside.
For the meringue: In a mixing bowl, beat egg whites and cream of tartar until soft peaks form. Gradually add erythritol and continue beating until stiff peaks form.
Spread the meringue over the lemon filling, sealing it at the edges. Bake for 10-15 minutes, until golden.
Let the pie cool for 1 hour at room temperature, then refrigerate for at least 2 hours before slicing.