Put oven rack in the middle position and preheat to 425°F. Line a large, rimmed baking sheet with foil, parchment or a silpat mat.
In a small bowl, stir together the sugar and cornstarch to get rid of any lumps. In a large bowl, toss the berries, sugar-cornstarch mixture, lemon juice, cinnamon and salt.
Unwrap the puff pastry onto the sheet, or roll regular pastry into a 10" (ish) circle. Mound the blueberries into the middle of the dough, leaving an inch or two around the edge. Fold the edges over the filling, just enough to keep the berries from sliding out. It can overlap and look rustic, there is no need for neatness.
Brush the pastry with egg and sprinkle with sugar. Bake until golden, 25-30 minutes. If it's browning too quickly, cover loosely with some foil. Cool for a few minutes before sliding out onto a cutting board to cut and serve. Serve warm, with whipped cream sweetened with maple syrup or vanilla ice cream.