Adapted from mrfood.com
(17.3-ounce) package frozen puff pastry, thawed
apples, cored, peeled, and diced
cup plus 1 tablespoon sugar, divided
teaspoon ground cinnamon
Preheat oven to 425 degrees F. Line a baking sheet with parchment paper. Unfold pastry onto a cutting board and cut into 4 equal squares. In a skillet over medium heat, melt butter. Stir in apples, 1/4 cup sugar, cinnamon, and cornstarch and cook 3 to 4 minutes, or until tender. Spoon about 2 tablespoons of apple mixture into center of each pastry square, leaving 1/2-inch border around edges. In a small bowl, whisk together egg white and water. Brush edges of each square with mixture. Fold each square into a triangle. Using a fork, press edges to seal. Brush top of each pastry with remaining egg white mixture and sprinkle with remaining sugar. Place turnovers on prepared baking sheet. Bake 12 to 14 minutes, or until golden brown.