Make the Pastry: In a large bowl, whisk together the flour and salt. Add the butter to the well and, using a pastry blender, cut it into the flour mixture until it resembles coarse meal. In a small bowl, whisk together the sour cream, lemon juice and water. Add to the flour mixture and, using a fork, gently toss and stir together until large lumps form and all of the flour has been moistened. Using your hands, pat the dough into a ball and wrap with plastic wrap. Refrigerate for 1 hour (if preparing ahead of time, the dough can be stored in the refrigerator for up to 2 days or in the freezer for up to 1 month).
Assemble the Pies: Line a baking sheet with parchment paper. Divide the refrigerated dough in half. On a lightly floured work surface, roll out one half of the dough to ⅛-inch thickness. Using a 4½-inch round biscuit cutter, cut seven circles out of the rolled dough. Transfer the circles to a parchment-lined baking sheet. Repeat the rolling and cutting process with the remaining half of dough. Refrigerate the dough circles for 30 minutes.
Meanwhile, in a small pan, combine the apples, sugar, cinnamon and salt, and cook over low heat for 8-10 minutes, stirring frequently, until the apples are tender. Set aside. Prior to preparing the pies, drain any accumulated liquid from the apples.
Remove the chilled dough from the refrigerator. Spoon about 1 to 2 tablespoons filling onto one half of each circle of dough. Brush a little water around the circumference of the dough, and fold it in half so the other side comes down over the filling, creating a semicircle. Seal the hand pie, and make a decorative edge by pressing the edges of the dough together with the tines of a fork. Refrigerate the assembled hand pies for 30 minutes.
Bake the Pies: Preheat oven to 375 degrees F. In a small bowl, whisk together the egg yolk and water for the egg wash. Remove the chilled hand pies from the refrigerator, and lightly brush with the egg yolk wash. Using a paring knife, cut a few small slits on the top of each hand pie, then sprinkle generously with coarse sugar. Bake until the hand pies are golden brown, anywhere from 20 to 40 minutes, depending on how thick you rolled the dough. Remove the pies from the oven, and let stand to cool slightly before serving. The pies are best eaten within a day of baking them, but they can be stored in airtight container at room temperature for up to 3 days. They can also be wrapped with plastic wrap and placed in a freezer storage bag and frozen for up to 2 months.
Feel free to use any type of apple you'd like in this recipe, as it's quite forgiving. I've used Granny Smith, Gala, and Honeycrisps!