Step 1: Prepare your cherry filling. In a medium saucepan, combine the pitted (or slightly thawed and drained) cherries, granulated sugar, lemon juice, and a pinch of salt. Cook over medium heat, stirring occasionally, until the cherries begin to soften and release their juices, about 5-7 minutes.
Step 2: In a small bowl, whisk together the cornstarch (or flour) with 2 tablespoons of cold water until smooth. Pour this slurry into the simmering cherry mixture, stirring constantly. Continue to cook for another 1-2 minutes, until the filling has thickened to your desired consistency. It should be thick enough to cling to a spoon. Remove from heat and stir in the vanilla extract and almond extract (if using). Let the filling cool completely. This is crucial; warm filling will melt your pastry and lead to soggy turnovers.
Step 3: Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.
Step 4: Prepare the pastry. Gently unfold the puff pastry onto a lightly floured surface or roll out your pie crust. Cut the pastry into eight equal squares (approximately 4x4 inches).
Step 5: Assemble the turnovers. Place a spoonful (about 2 tablespoons) of the cooled cherry filling onto one half of each pastry square, leaving a small border around the edges. Be careful not to overfill.
Step 6: Fold the pastry over the filling to create a triangle. Gently press the edges together to seal. Use the tines of a fork to crimp the edges securely, creating a classic turnover look.
Step 7: Make the egg wash. In a small bowl, whisk together the egg and 1 tablespoon of water (or milk). Lightly brush the tops of the turnovers with the egg wash. This will give them a beautiful golden sheen.
Step 8: Sprinkle with coarse sugar (optional). If desired, sprinkle a little coarse sugar over the tops of the egg-washed turnovers for added sparkle and crunch.
Step 9: Bake! Place the turnovers on the prepared baking sheet. Bake for 18-22 minutes, or until the pastry is puffed, golden brown, and cooked through. The exact baking time can vary based on your oven and the thickness of your pastry.
Step 10: Cool and glaze. Carefully transfer the baked turnovers to a wire rack to cool slightly. While they cool, prepare the optional glaze by whisking together powdered sugar, milk (or lemon juice for a tangier glaze), and vanilla extract until smooth. Drizzle over the warm turnovers. Let the glaze set before serving.