Set oven to 400F and line two baking sheets with parchment paper. Peel your apples then cut the flesh from the core and dice into smaller pieces about 1/4 inch thick.
Melt butter in a large pan over medium heat then add the apples and cook for a few minutes while stirring frequently. Add the brown sugar, salt, cinnamon, and lemon juice. Mix to coat apple pieces and cook for about 5 minutes or until the apples are softened and have released juice then reduce heat to low.
Sprinkle in the corn starch in and stir until the juice thickens up a bit then transfer to a bowl and refrigerate to chill.
Roll your thawed puff pastry out on a lightly floured surface into a 12x12 inch square then cut four into equal squares then repeat the process for the other sheet.
Place a few tablespoons of filling in each square. You should Imagine a diagonal line running across the square and add the filling to one side of that leaving a half inch border.
Mix an egg with a tablespoon of water, milk, or cream then brush it over the edge of the square. Fold the puff pastry onto itself then use a fork to seal the edge. Brush the top with egg wash and sprinkle with sugar. I use sanding sugar but you can use turbinado or regular granulated sugar.
Bake at 400F for about 18-20 minutes or until golden and puffed.
Make a simple icing by combining powdered sugar with a tablespoon of water and a teaspoon of vanilla in a bowl and mixing until smooth. You can add a bit more water if the consistency needs to be thinned out. Drizzle over the warm turnovers and enjoy!