Preheat oven to 375º Fahrenheit. Prepare a 9 inch round tart pan according to manufacturers instructions, buy spraying with coconut oil or lining with parchment, if desired.
Place almond flour, granulated sweetener, baking powder, and sea salt in a food processor. Pulse to combine.
Add vanilla extract and butter to the food processor. Pulse until the mixture looks mealy and it sticks together. Transfer dough to the prepared pan. Using you fingers, gently press the dough into the pan. If you're using a pie pan, flute the edges if you want.
Set pastry aside while you prepare the filling.
Place cream cheese in a medium bowl. using a hand mixer on low speed, blend the cheese until smooth. Mix in the granulated sweetener at low speed. When this mixture is smooth, blend in the egg, lemon extract, and lemon zest at low speed until combined, then whip at high speed until mixture is smooth and fluffy.
Fill the prepared tart shell with the filling and spread to even out the filling.
Place pan on a cookie sheet (in case of leakage) and bake tart in the preheated oven for 13-18 minutes, or until the shell and the top of the filling begin to brown. The filling may still jiggle when it is done. If the top bubbles up, don't worry, the bubble will settle on cooling.
Remove tart from the oven and place on a cooling rack. Allow to cool for 20 minutes, then cover and refrigerate.
Add granulated sweetener and xanthan gum to a 1 1/2 quart saucepan. Stir together to combine. Stir in lemon juice until mixture is smooth. Add water, one tablespoon at a time, stirring mixture until smooth after each addition. Stir in blueberries.
Place saucepan over medium-low heat. While stirring frequently, bring mixture to a simmer. If using fresh blueberries, they will start to release their pigments at this time. Simmer for 30 seconds to one minute, then remove from heat. Allow to cool to room temperature before proceeding.
Pour topping over the filling. Cover the tart and refrigerate for at least one hour before removing from the pan and serving.
Serving size: 1/12th of the pie.
Fat (g.): 15
Carbs (g.): 5
Fiber (g.): 2
Protein (g.): 4
Net carbs (g.): 3