• In a large bowl, combine the lemon juice and 4 cups of water.
• Place the apple slices into the lemon water to prevent browning.
• Melt butter in a large skillet over medium heat.
• Drain the apples and place them into the skillet.
• Cook and stir apples for 2 minutes.
• Add the brown sugar and cinnamon to the skillet. Stir and cook for 2 more minutes.
• In a small bowl, mix the cornstarch and 1 tablespoon of water until smooth.
• Pour the cornstarch mixture into the skillet and stir well.
• Cook for 1 minute, until the sauce thickens. Remove the skillet from the heat and let the apples cool slightly.
• Preheat the oven to 400°F (200°C).
• Unfold the puff pastry sheets. Press together any cracks.
• Trim each sheet into a square. Then cut each square into 4 smaller squares.
• Spoon the apple mixture into the center of each smaller square.
• Fold the pastry from corner to corner to form a triangle and press the edges to seal.
• Place the turnovers on a baking sheet, leaving about 1 inch of space between each one.
• Bake the turnovers for 25 minutes, until puffed and lightly browned.
• Let the turnovers cool completely before glazing.

For the Glaze:

• In a small bowl, mix the powdered sugar, milk, and vanilla extract to make the glaze.
• If needed, adjust the thickness by adding more powdered sugar or milk.
• Drizzle the glaze over the cooled turnovers.