Pulse flour, sugar, and salt in a food processor. Add the butter and pulse until texture is coarse. Add ice water and pulse until the dough comes together in clumps; add more water if needed. Form the dough into a ball, wrap in plastic, and chill until firm, about 1 hour.

Preheat oven to 350 degrees. Line a baking sheet with parchment paper. Prepare the filling by mixing together the blueberries, lemon zest and juice, sugar, vanilla, cornstarch, and salt.

Roll out the dough on a floured surface to an 18" x 10" rectangle (you may need to let the dough warm up a bit if it's too hard to roll). With a pizza slicer, cut the dough into 6 rectangles, approximately 3" x 10" each. Evenly distribute the blueberry mixture between the 6 rectangles by mounding it on the bottom half. With your fingers, wet the edges of the dough with water and gently fold the top half over the blueberry mound, sealing the edges tightly. Use a fork to crimp the edges. Then place the hand pies onto the baking sheet. Brush them with the egg wash and sprinkle with sugar. Cut slits in the tops.

Bake the hand pies until crust is golden brown and the juices are bubbly, about 35-40 min. It's OK that the juices run onto the parchment paper. Best served warm.