Prepare the Apple Filling

1. Cook the Apples: In a medium saucepan over medium heat, add the diced apples, brown sugar, cinnamon, and lemon juice. Cook for about 5-7 minutes, stirring occasionally, until the apples soften slightly.
2. Thicken the Mixture: Stir in the cornstarch slurry and cook for another 2 minutes, until the filling thickens. Remove from heat and let it cool.

Assemble the Turnovers

1. Preheat the Oven: Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper.
2. Roll Out the Pastry: On a lightly floured surface, roll out the thawed puff pastry and cut into 8 equal squares.
3. Fill the Turnovers: Place a spoonful of the cooled apple filling in the center of each square. Fold each square into a triangle, pressing the edges with a fork to seal.
4. Brush with Milk and Sprinkle Sugar: Lightly brush the tops of the turnovers with plant-based milk and sprinkle with coarse sugar if desired.

Bake the Turnovers

1. Bake Until Golden: Transfer the turnovers to the prepared baking sheet and bake for 20-25 minutes, or until the pastry is golden and puffed.
2. Cool Slightly: Let the turnovers cool for a few minutes on the baking sheet before transferring to a wire rack.