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 12×12 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.
This is a great make ahead and freeze dessert. You can bake the frozen turnovers without thawing so they’re great for last minute treats.
Try adding different spices to the filling. Cardamom, allspice and nutmeg will work.
If you don’t have brown sugar you can use granulated instead.
Try adding a tablespoon of caramel with the filling while assembling or drizzling it on top instead of the icing.
If you’re in a rush you can mix the apples with sugar, salt and spices then bake raw. They will soften while baking but they may need a few more minutes.