Preparation

1. In a saucepan, off the heat, combine the sugar and cornstarch. Add the blueberries or raspberries and lemon juice. Bring to a boil over medium heat and simmer for 3 to 4 minutes. Transfer to a bowl. Place plastic wrap directly on the fruit. Let cool and refrigerate until completely chilled, about 2 hours.
2. With the rack in the lowest position, preheat the oven to 200 °C (400 °F). Line a 43 x 30-cm (17 x 12-inch) baking sheet with parchment paper.
3. On a floured work surface, roll out 454 g (1 lb) of the dough into a rectangle the same size as the baking sheet. Place the dough on the baking sheet and cover with parchment paper. Roll out the remaining dough and cut into 14 1-cm (½-inch) wide strips with a serrated wheel or knife. Place on the parchment paper. Refrigerate for 30 minutes.
4. In a small bowl, combine the egg and water with a whisk. Set the egg wash aside.
5. Set the parchment with the strips aside. Spread the fruit filling evenly in the centre of the rectangle to 2-cm (¾-inch) of the sides. With a pastry brush, brush water around the edges of the dough. Place, diagonally, half of the dough strips, side by side, spacing them evenly. Lightly press the strips around the perimeter. Repeat with the remaining strips in the opposite direction. Trim the edge of the pastry to remove excess strips. Brush the dough with the egg wash and sprinkle with coarse sugar, if desired.
6. Bake for about 35 minutes or until the pastry is golden brown. Let cool completely.