Line a baking tray with parchment (baking) paper.

On a lightly floured cold surface, roll out the puff pastry dough into a 10x15-inch sheet. Cut horizontally to form two 5x15-inch strips. Then, cut each strip into three equal portions to get a total of six 5-inch squares. Place the squares on the prepared tray, cover and refrigerate for 20-25 minutes.

Meanwhile, melt the butter and lightly sauté the apples in a shallow saucepan. Cover and cook on medium-low heat until the apples are soft, about 5 minutes. Add the sugar and the lemon juice and cook until the extra water evaporates, about 2-3 minutes. Remove from heat and let the filling cool.

Leaving the edges, brush the center of the puff pastry squares with some egg wash. Drop about 1-tbsp of the apple filling in the center of each square. Fold over the squares diagonally and press to seal the edges. Cover and refrigerate the turnover for at least 30 minutes.

Preheat the oven to 190°C/375°F. Brush the turnovers with egg wash and bake until golden brown, about 30-35 minutes. Transfer the turnovers to a wire rack and let cool for 15 minutes and serve warm.