In a small bowl, stir together the cornstarch, water, and lemon juice until the cornstarch is dissolved. 1 tablespoon water 1 tablespoon lemon juice 2 tablespoons cornstarch
In a medium saucepan, over medium heat, combine the cornstarch mixture, blueberries, and sugar, and cook until mixture has thickened and bubbles, about 5-7 minutes. Remove from heat and set aside to cool. 3 cups blueberries ⅓ cup (67 g) granulated sugar
On a lightly floured surface, use a lightly floured rolling pin to roll the first sheet of thawed puff pastry to an 10" square. With a pizza cutter or sharp knife, cut into 4 equal-sized squares. Place 2-3 tablespoons of the berry mixture in center of each square, leaving at least a 1/2" border. 1 lb puff pastry 2 sheets
Thoroughly beat together 1 egg and 1 Tbsp water. Lightly brush egg wash lightly on edges of the pastry (just enough to seal), bring the edges together and crimp tightly along the edges with a fork to seal. Save the rest of the egg wash for brushing on right before baking. 1 large egg+ 1 Tbsp water for egg wash
Transfer to a parchment-lined baking sheet, spaced about 1" apart. With a sharp knife, cut 2-3 small slits in the top of each turnover. Refrigerate for 20 minutes (this helps the filling from leaking out and turnovers to rise up tall.)
Position a rack in the center of your oven and preheat oven to 400oF (205oC). Brush the tops of each turnover with egg wash and bake 20-23 minutes or until golden brown and puffed.
While turnovers are still warm, stir together powdered sugar, vanilla, and milk. Adjust consistency if needed. Drizzle glaze over turnovers. ½ cup (57 g) powdered sugar 1-2 Tablespoons milk ½ teaspoon vanilla extract