Starting with the Crust, in a bowl combine flour, Natvia and salt. Cut butter cubes using a fork or pastry blender until dough resembles coarse meal. Beat egg yolk with cold water and add it slowly, until dough comes together.
You may not need all the liquid. Turn the dough onto a floured surface and roll into a disk. Wrap in plastic and refrigerate for 30 minutes.
Moving on the the tart, preheat oven to 200C. Line a baking sheet with parchment paper, slice gala apple into thin slices. Toss in lemon juice if desire.
Mix the egg yolk and water for the egg wash together and set aside.
Place the crust on the baking sheet. Arrange apple slices and sprinkle with cinnamon and sugar. Brush the surface with egg wash.
Bake for 25-30 minutes, until lightly golden brown. Remove from oven and let cool for 5 minutes. Serve hot, preferably with vanilla ice cream.