On a lightly floured work surface, roll out the pastry to a 12-by-16-inch rectangle and cut out twelve 4-inch squares. Transfer the squares to a baking sheet and freeze until firm, 10 minutes. In a bowl, toss the apples with the sugar, farmer cheese, cinnamon, salt and 1 1/2 tablespoons of flour.
Preheat the oven to 375°. Line 2 baking sheets with parchment paper. Brush the edges of the squares with the egg and mound 2 tablespoons of the apple filling on each one. Fold to form triangles and press to seal. Freeze until firm, about 15 minutes.
Using a fork, crimp the edges of the triangles. Brush the tops with egg and sprinkle with sugar. Cut 3 small slits in each to release steam. Bake until golden and crisp, about 30 minutes, shifting the pans halfway through. Let cool, then serve.