01 -
Heat the oven to 400 degrees F. Line two baking sheets with parchment paper.
02 -
Melt the butter in a skillet over medium heat. Add the diced apples, brown sugar, lemon juice, cinnamon, nutmeg, and salt. Cook for 6-8 minutes, stirring often, until the apples begin to soften.
03 -
Stir in the cornstarch slurry and cook for 1-2 minutes until the juices turn glossy and thick. Remove from the heat, stir in the vanilla, and spread the filling on a plate to cool for about 15 minutes.
04 -
On a lightly floured surface, gently roll each puff pastry sheet to smooth the seams. Cut each sheet into 4 squares for 8 total squares.
05 -
Whisk the egg with 1 tablespoon water. Spoon about 2 tablespoons cooled apple filling onto each pastry square. Brush the edges with egg wash, fold into triangles, press out air, and crimp the edges firmly with a fork.
06 -
Place the turnovers on the prepared baking sheets. Cut a small slit in the top of each one and chill for 15 minutes. Brush the tops with egg wash and sprinkle with coarse sugar if using.
07 -
Bake for 20-25 minutes, rotating the pans halfway through, until the pastry is puffed and deep golden brown. Cool on the pan for 10 minutes.
08 -
Whisk the powdered sugar, milk, and vanilla into a pourable glaze. Drizzle over the warm turnovers and let set for a few minutes before serving.