Preheat oven to 425 F. Line a baking sheet with parchment paper; set aside.
In a large bowl, toss the berries with the sugar, cornstarch, lemon juice, cinnamon, and salt.
Unroll the puff pastry onto the baking sheet. Mound the blueberries into the middle of the dough, leaving an inch or two around the edges. Fold the edges over the filling to keep the berries from sliding out, overlapping and pinching where needed. Brush the edges of the pastry with egg and sprinkle with the course sugar.
Bake until golden, about 20-25 minutes. Cool for 10-15 minutes before serving.