Preheat the oven to 450F

In a small saucepan, combine 1/2 cup only of blueberries, cornstarch, sugar, and lemon zest under low heat. Crush the blueberries completely and bring to a boil, then remove from heat

Add butter and remaining blueberries to sauce and mix thoroughly

Cut each sheet of puff pastry into quarters. Spoon 2 tablespoons of blueberry filling into each, then fold over diagonally and seal the edges

Beat together egg and water, then brush pastry tops with mixture. Puncture with a fork or toothpick for ventilation

Bake at 450F for 15 minutes.