Preparation Steps

• Line a large rimmed baking sheet with parchment and place it in the freezer to chill. Keep puff pastry in the fridge until needed so it stays cold.
• Make the filling: Add blueberries, granulated sugar, and lemon zest to a medium saucepan. Warm over medium heat until the berries release juices and begin to simmer, 2–3 minutes. In a small bowl whisk cornstarch with 1½ tbsp water, then stream the slurry into the bubbling fruit while stirring. Cook 1–2 minutes more, just until glossy and thickened. Scrape into a shallow bowl and refrigerate 15 minutes to cool.
• Preheat the oven to 415°F with a rack in the upper-middle position. Whisk the egg with 2 tsp water to make an egg wash.
• Working with one sheet of pastry at a time on a lightly floured surface, roll into a 10×12-inch rectangle. Cut into four 5×6-inch rectangles. Spoon about 2 tablespoons of cooled blueberry filling onto one half of each rectangle, leaving a clean border.
• Brush the bare edges lightly with egg wash. Fold the pastry over the filling to form a 5×3-inch packet, pressing out air. Crimp edges with a fork to seal well. Transfer to the chilled baking sheet and repeat with the second pastry sheet to make 8 turnovers. Cut 2 small vents on top of each.
• Freeze the assembled turnovers for 10 minutes. Brush tops with more egg wash and, if using, sprinkle with turbinado sugar.
• Bake until puffed and deeply golden, 18–20 minutes, rotating the pan once if needed. Cool on the pan for 5 minutes, then move to a rack to cool another 10 minutes.
• Glaze: Stir powdered sugar with 1 tbsp milk until smooth and thick but pourable, adding a few drops more milk only if necessary. Drizzle over the just-warm turnovers and let set before serving.