In a heat proof bowl, combine egg, egg yolks, lemon zest, juice and powdered erythritol or Swerve.
Whisk to combine. Fill a large pot with water to create a double boiler.
Heat over medium heat. Place the heat proof bowl above it. Whisk constantly for 8-9 minutes, until the mixture thickens.
Strain through a fine mesh strainer and add the butter. Let it cool to room temperature, then cover and refrigerate or fill the tart crust with it.
Preheat oven to 350 F. Combine all ingredients in a bowl and mix with a spatula to combine.
Press down the bottom and about 1/2-inch up the walls of a 9-inch tart pan. Using a fork, press to make holes on the bottom of the crust.
Bake at 350 F for 13-15 minutes, until golden. Cool completely.