Melt 1 tbsp butter in a saucepan over medium heat. Add the diced apples, 3 tbsp brown sugar, 1 tsp cinnamon, and 1 tsp lemon juice. Cook for 5 minutes, stirring, until apples soften. Stir together 1 tbsp cornstarch and 1 tbsp cold water, pour into the pan, and stir for 1 minute until thickened. Remove from heat and cool for at least 10 minutes.

Heat oven to 400°F (200°C) and line a baking sheet with parchment. Lightly flour your surface, unroll the puff pastry, and cut into 4 equal rectangles (roughly 12cm x 10cm each).

Spoon 2 to 3 tbsp of cooled filling onto one half of each rectangle, leaving a 1.5cm border. Fold the pastry over, press the edges firmly with a fork to seal, and cut 2 to 3 small vents across the top.

Beat 1 egg with 1 tbsp milk and brush over the tops and edges of all 4 pastries. Bake on the middle rack for 20 to 22 minutes, until deep amber and puffed.

Cool on a wire rack for 5 minutes, then sift 2 tbsp powdered sugar over the tops and serve warm.