1¾ cups (254 grams) chopped fresh nectarines (about 2 medium nectarines)½ cup (100 grams) granulated sugar2 teaspoons (10 grams) fresh lemon juice¼ teaspoon kosher salt2 tablespoons (16 grams) cornstarch3 tablespoons (45 grams) water, divided½ teaspoon (2 grams) vanilla extractAll-purpose flour, for dustingRough Puff Pastry Dough (recipe follows)1 large egg (50 grams)
In a small saucepan, stir together nectarines, sugar, lemon juice, and salt. Cook over medium heat, stirring frequently, until fruit begins to break down and release juice, about 5 minutes.
In a small bowl, whisk together cornstarch and 2 tablespoons (30 grams) water until smooth. Add cornstarch mixture to nectarine mixture, and cook, stirring frequently, until mixture is thick and translucent, 2 to 3 minutes. Remove from heat, and stir in vanilla. Let cool completely.
Preheat oven to 400°F (200°C). Line 2 baking sheets with parchment paper.
On a lightly floured surface, roll Rough Puff Pastry Dough into a 16×12-inch rectangle. Cut into 12 (4-inch) squares. Spoon 1½ tablespoons (30 grams) nectarine mixture into center of each square.
In another small bowl, whisk together egg and remaining 1 tablespoon (15 grams) water. Using a pastry brush, brush edges of dough with egg wash. Fold squares in half diagonally, and crimp edges to seal. Place at least 1½ inches apart on prepared pans. Freeze until firm, about 15 minutes.
Using a small knife, cut 3 small vents in top of each pastry. (See Pro Tip.) Brush with egg wash.
Bake for 10 minutes. Reduce oven temperature to 375°F (190°C), and bake until golden brown, 10 to 15 minutes more. Let cool on pans for 10 minutes. Remove from pans, and let cool on wire racks for at least 15 minutes. Serve warm or at room temperature.
PRO TIP: To ensure the turnovers rise evenly and don’t lose filling, gently score the fold of frozen turnovers with a sharp knife (without cutting all the way through). This will allow the pastry to rise and puff without popping open.