1. Thaw puff pastry according to package instructions.
2. Preheat oven to 410F.
3. Peel and trim apples, then cut into pieces.
4. In a medium bowl, toss apples with brown sugar and cinnamon.
5. In a skillet over medium-high heat, melt butter and add apples.
6. Brown the apples, then cover the pan and lower the heat to medium to allow the apples to caramelize gently until they have softened, about 8-10 minutes.
7. Meanwhile, roll out the dough and cut into 8 equal wedges using a pizza cutter or small knife.
8. Arrange apple pieces on the wide ends of the wedges, about 1cm from the edge.
9. With a knife, cut the tips into two pieces and bring them back to the apples on the outside.
10. Fold in the protruding piece of dough next to the apples.
11. Carefully place turnovers on a baking sheet lined with parchment paper or silicone sheet.
12. Brush pastry with egg yolk.
13. Bake in center of oven for about 20-25 minutes, or until golden brown.