Bring the frozen puff pastry to room temperature. Cut into 4 equal square pieces. Peel the apple and chop into 1 to 1.5 cm pieces.
Melt the butter in a sauce pan. Add the apples, lemon juice, and sugar and slowly simmer over low heat while stirring until the liquid boils out. Be careful not to burn it.
Sprinkle cinnamon powder on the apples, stir, then remove from the heat. You should have about 1 and 1/2 cups (300 ml) of apples.
Place 1/4 of the stewed apples onto the puff pastry, then fold it into a triangle.
Use a fork to crimp the edges.
Brush with egg wash, then sprinkle sugar. Bake for about 20 minutes in an oven preheated to 200C until the surface is golden brown.
I used Granny Smith apples, which are very tart and crisp. You could also use Jonathan apples as well.