Preheat oven to 375°F (190°C). Line a baking sheet with parchment paper.
Make the filling: In a saucepan, combine blueberries, sugar, and lemon juice. Cook over medium heat until berries release their juice. Add cornstarch slurry and stir until thickened. Remove from heat and let cool.
Prepare the pastry: Roll out puff pastry slightly. Spoon the blueberry filling down the center.
Fold and seal: Cut slits along both sides of the pastry and fold strips over the filling, alternating sides to create a braid pattern. Brush with egg wash.
Bake for 25–30 minutes, or until golden brown.
Glaze: Mix powdered sugar with milk or lemon juice. Drizzle over the cooled strudel before serving.