Thaw the frozen puff pastry according to the package instructions. Being sure that it stays cold.
Toss the diced apples with the fresh lemon juice, set aside.
In a medium-sized saucepan over medium heat, melt the butter. Add the diced apples and cook until they are softened about 5 minutes.
Reduce the heat to low and stir in the dark brown sugar, cinnamon, nutmeg, and salt. Simmer for another 3 to 5 minutes until the apples are soft. Remove from the heat and let cool.
Preheat the oven to 400 degrees F. Prepare a large baking sheet (or two) with parchment paper.
Lightly flour a surface and roll out the first pastry sheet to an 11-inch square. Cut the pastry into four equal-sized (about 5 1/2 inches) squares.
Spoon about 1 1/2 – 2 heaping tablespoons of the apple mixture onto the center of each square down the long side of the triangle leaving at least a 1/2″ border.
Beat together the egg and water. Brush the egg wash lightly on all four edges of each pastry square.
Bring the edges together to form a triangle, press the edges together slightly. Then crimp the edges with a fork to seal tightly.
Transfer to the prepared baking sheet, leaving at least one-inch space between each turnover.
Refrigerate for 20 minutes. This helps the dough to chill again and helps keep the filling from leaking out.
Cut 2 or 3 small slits in the top of each turnover, then brush each one with egg wash.
Bake for 20-24 minutes or until golden brown and puffed.
Remove from the oven to a wire rack to cool at least 10-15 minutes before drizzling with the glaze.