To a food processor, add the walnuts, toasted coconut, 3 tbsp powdered erythritol, and salt. Pulse until a fine powder forms. Add the melted butter.
Pulse again until the mixture sticks together. Transfer the crust mixture into a pie pan. Press into the bottom and sides, then transfer to the freezer while preparing the lemon filling.
To a bowl, add the eggs, egg yolks, and lemon juice. Whisk to combine. Transfer the bowl over a double boiler and keep whisking.
Keep the heat on low and add ¾ cup erythritol and xanthan gum and whisk for another 3-4 minutes. Once the mixture thickens, add the butter and whisk until incorporated. Take the bowl off the heat and stir in the lemon extract.
Pass the mixture through a sieve into a clean bowl to get rid of the lumps. Meanwhile, add the heavy cream to another bowl. Beat until stiff peaks using a hand mixer.
Add the whipped cream to the lemon mixture. Fold gently until incorporated. Take the crust out of the freezer and pour the lemon filling over the crust.
Using a spatula, spread the filling and flatten the surface. Transfer the tart to the freezer for 1 hour, or refrigerate for 4-6 hours. Cut it into 8 slices, and serve.