Preheat oven to 400ºF (200ºC).
Place the apples, lemon juice and sugar in a pot over medium heat and cover with a lid.
Toss to turn a few times, but only steam them for a few minutes, just to soften them a little. Remove from heat and allow to cool.
Roll one sheet of puff pastry into a square and cut into 4 squares (repeat with the other one afterwards-keep it in the fridge for now.)
On one piece of puff pastry, put about 2 tablespoonfuls of apple filling. Imagine that there is a line diagonally across the pastry and keep the filling on one triangle of the square.
Wet two edges of the puff pastry with your finger dipped in water. Then fold the empty corner of the pastry over the apples and seal the edges with a fork. With a knife, make a small slit or two in the top of the pastry.
Repeat with the rest of the apples and pastry, and place on lined baking sheet. Brush with a very slightly beaten egg white (do not let it get frothy, just to make it easier to brush), and sprinkle with a coarse sugar. Another option is to omit the sprinkled sugar and just dust the apple turnover with powdered sugar after baking.
Bake for approximately 20-25 mins. until golden brown.
Allow to cool if filling with fresh cream or the cream will melt. Slice the top part of the pastry open, without cutting the top off completely.
Fill with freshly whipped, unsweetened cream. (Add a dusting of powdered sugar if you like.)