To make the pastry:
Cut the butter into cubes and chill it in the freezer for about 30 minutes. Combine the flour, kosher salt, and 1 tablespoon of the sugar in a food processor and mix for about 5 to 10 seconds.
Scatter the cold butter cubes on top of the flour and pulse about 10 times, just until the butter is the size of peas.
Pour the cold water over the top of the flour and process for about 10 to 15 seconds, just until the water is blended into the flour and butter and it starts to gather. Don’t over process or your pastry will be tough. The dough will still be crumbly.
Turn the pastry dough out onto a lightly floured sheet of parchment paper. Using the parchment paper, gently pull the dough together and press it into a disc.
Gently roll the dough into a round that’s about 15” to 16” and 1/8” in thickness, turning occasionally and adding a light dusting of flour underneath so it doesn’t stick to the paper. Don’t worry if it’s not perfectly round, this is rustic!
Slide the parchment sheet with the pastry dough onto a rimmed baking sheet. Trim any excess paper that’s hanging over the edge of the pan.
To make the filling:
In a medium mixing bowl, whisk the flour, brown sugar, cinnamon, and nutmeg until well blended.
Measure out 8 cups of the apple slices. Depending on the size of the apples you might have some leftover. Combine the apples with the flour and spice mixture along with the vanilla extract. Mix well, coating all the apple slices as evenly as possible.
To make the streusel topping:
Using a fork or pastry cutter, blend the butter together with the brown sugar, vanilla powder, cinnamon, and flour. The topping will be crumbly.
To assemble the tart:
Pile the apples in the center of the pastry dough, leaving an edge that’s about 3” around the apples.
Scatter the pieces of butter over the apples and sprinkle with the streusel topping.
Take the edge of the pastry and fold up over the apples. Then continue folding, turning the edges of the dough up over the apples toward the center. The pastry won’t completely cover the apples. Take care not to break the pastry or poke holes in the dough.
In a small bowl whisk the egg and 1 tablespoon milk together. Using a pastry brush, lightly brush the egg wash over the pastry dough then sprinkle with the remaining teaspoon of sugar.
Bake the tart in a 375-degree degree for about an hour, or until the crust is golden brown and the apples are tender when pierced with the tip of a sharp knife. Let the tart cool on a wire rack for about 10 to 15 minutes before slicing to allow the juices to set.