Preheat an oven to 400 degrees F (200 degrees C). Line a baking sheet with parchment paper.
Whisk together the flour, salt, cream of tartar, baking soda, sugar, and apple pie spice in a mixing bowl. Cut in the butter with a knife or pastry blender until the mixture resembles coarse crumbs. Add the sourdough starter and mix by hand to form a soft dough.
Turn the dough out onto a lightly floured surface and divide it into 4 pieces. Pat or roll one piece of dough into a 1/2-inch thick round. Cut it into 4 wedge-shaped pieces and place the scones on the prepared baking sheet about 1 inch apart. Repeat with remaining pieces of dough. Brush the tops of the scones with milk and sprinkle with coarse sugar, if desired.
Bake until the scones just start to turn golden, 12 to 15 minutes.
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