1. Mix the all-purpose flour, granulated sugar, and salt in a bowl. Cut in the cold, cubed unsalted butter until the mixture resembles coarse crumbs. Stir in the sour cream until a soft dough forms. Shape the dough into a ball, wrap it in plastic, and chill it in the refrigerator for an hour.

2. Prepare the apple filling by combining the diced apples, brown sugar, cinnamon, nutmeg, and lemon juice in a pan. Cook over medium heat for 5-7 minutes until the apples are tender and the mixture has thickened slightly. Allow the filling to cool.

3. On a lightly floured surface, roll the chilled dough into a large rectangle. Spread the apple filling down the center of the dough, leaving space on the edges. Fold the long sides of the dough over the filling, pinching the seams to seal. Shape the filled dough into an oval or horseshoe on a parchment-lined baking sheet.

4. Preheat your oven to 375°F and bake for 25-30 minutes, or until golden brown.

5. Whisk together the powdered sugar, milk, and vanilla extract to create a glaze. Drizzle over the warm pastry.