Preheat the oven to 350°F and lightly grease an 8-inch pie dish or tart pan.

In a mixing bowl, combine the melted butter, almond flour, and 1 tablespoon granulated sweetener for the crust and stir until the mixture resembles damp sand.

Add the 1 medium egg for the crust and mix until a thick, cohesive dough forms.

Press the crust mixture firmly and evenly into the bottom and up the sides of the prepared pie dish, compacting the corners so they do not crumble.

Bake the crust for 10 to 12 minutes, until the edges are just turning golden, then remove and let cool completely on a rack.

In a medium saucepan off the heat, whisk together the heavy cream, 3 eggs for the filling, 3 egg yolks, 2 tablespoons granulated sweetener, lemon zest, and lemon juice until smooth and well combined.

Place the saucepan over low to medium-low heat and cook, stirring constantly with a silicone spatula, scraping the bottom and sides, until the mixture thickens enough to coat the back of the spatula and a line drawn with your finger stays clear.

Remove the pan from the heat and continue stirring for 30 seconds to cool slightly and smooth the custard.

Pour the warm lemon filling into the fully cooled crust and smooth the surface with a spatula, then let it stand at room temperature while you prepare the meringue.

Reduce the oven temperature, if needed, to ensure it is at 350°F.

In a clean, dry mixing bowl, add the 3 egg whites for the meringue and a pinch of salt, then beat with an electric mixer on medium speed until frothy.

Add the vanilla flavoring and continue beating on high speed until soft peaks form.

If using, sprinkle in 1 tablespoon granulated sweetener for the meringue, a little at a time, while beating, until stiff, glossy peaks form that stand straight up without drooping.

Spoon the meringue over the warm lemon filling, spreading it to the edges of the crust to seal the pie and prevent gaps, then use the back of the spoon to create swirls on top.

Bake the pie for 10 to 12 minutes at 350°F, until the meringue is lightly golden on the tips and edges.

Remove the pie from the oven and cool at room temperature until just barely warm, then refrigerate for at least 2 hours before slicing and serving.