Start by making the tart dough. In a mixing bowl, use a hand mixer or whisk to combine the butter, sugar, salt, and egg yolks until fluffy.
Beat in the flour and milk until well-combined.
Divide the dough into 12 approximately equal pieces. Roll each piece into a ball, flatten the balls into disks, and cover the disks with a damp towel while you prepare the filling.
In a medium bowl, stir together the yogurt, cinnamon, lemon juice, and flour. Add the apples and pistachios and stir until completely coated with the yogurt mixture. Set aside.
Preheat the oven to 375º F and line two baking sheets with parchment paper.
Dust a clean work surface with flour. Roll out each disk of dough until about 1/8 inch thick. They should be about 6 inches in diameter but don't have to be perfectly circular. It's alright if the edges crack – it just makes the tarts look more rustic.
Add about 1/4-1/3 cup of filling to the center of each tart, leaving plenty of room along the edges. Fold the edges up and over the filling, pleating the folds so they overlap.
For the topping, beat the egg with the water and brush over the tops of the crusts (not on any exposed filling). Mix the brown sugar and cinnamon in a small bowl, then sprinkle on each tart on top of the egg wash. Try not to get too much brown sugar on the parchment since it can burn.
Arrange six tarts on each prepared baking sheet. Bake for 10 minutes at 375º.
Turn the oven down to 350º and bake for an additional 20-25 minutes, or until the tops of the tarts are golden brown.
Serve warm with vanilla ice cream or whipped cream.