For the dough:
In a bowl, mix the sour cream and sugar and set aside.
In a large bowl, mix the flour and salt.
Add the butter pieces and with a pastry blender or your fingers, mix the butter until the mixture is crumbly and has a flaky texture
Add the sour cream mixture and mix gently with a fork or your hands.

Gather the dough into a ball and divide it into two equal parts that you will flatten into two disks that you will wrap in plastic wrap and refrigerate for 1 hour.

For the filling:
Mix the flour, sugar, and cinnamon in a bowl.
Add the apples and mix well to coat them evenly.

Assembly:
Preheat the oven to 375 F (190 C) and position the rack in the middle.
Line two baking sheets with silicone mats or parchment paper.

Roll out the dough to a thickness of 1/8 inch (3 mm).
Cut out circles of 4 inches (10 cm) with a cookie cutter, then pass a little of the rolling pin over them to give them an oval shape.

Fill with one to two tablespoons of filling and sprinkle with a few pieces of butter.
Moisten the edge of the dough with water and fold in half.
Seal the edges by pressing with the tines of a fork.

Glaze the turnovers with the beaten egg mixed with a little water, then sprinkle with sugar.

Bake the turnovers for 20 minutes or until they are golden brown.
Let cool for 5 to 10 minutes before serving.