Pastry - Put the flour, salt, and sugar in a large bowl and mix together.
Add the butter and smoosh with your fingers, making a crumbly texture.
Make a well in the middle, and pour in the vanilla and water, starting with 2 Tbsp of water and incrementally add more water if the dough is too crumbly. You should be able to squeeze it in your hand and have it stay together but be a bit crumbly.
Quickly work in the flour to create dough. Do not over-mix.
Pat into a round disk, wrap in plastic, and refrigerate at least 30 minutes.
Heat the oven to 450°F/230°C. Roll the pastry into a round and lay on a baking stone with sides or baking sheet.
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For the filling - Stir together the sugar, flour, and cinnamon in a medium mixing bowl.
Toss in the apples to coat. Let sit for 10 minutes. Dump onto the pastry.
Dot apples with the butter.
Bring the edges of the pastry up over so they lay in, rough-edged, on the apples. They won't cover the apples completely.
Dot the pastry with some butter and sprinkle with sugar (coarse sugar makes the pastry crispy and sweet)
Bake until the crust is crisp and golden and the apples soft to a fork and caramelized, 40 to 45 minutes.