Preheat the oven to 425°F and line two baking sheets with parchment paper or Silpat baking mats.
Lightly flour your work surface then unroll the pie crusts. Using a 3- or 4-inch circular cookie cutter, cut out circles from each crust, re-rolling and re-cutting the scraps as needed.
Arrange half of the pie crust circles on the baking sheets, spacing them at least 1 inch apart. Set the remaining pie crust circles aside.
In a medium bowl, stir together the blueberries, sugar, lemon zest and lemon juice. Divide the blueberry mixture among the pie crust circles on the baking sheets, centering the filling in each circle and leaving a 1/4-inch space around the edges.
Top each pie crust circle with an additional pie crust circle, using a fork to crimp together the edges. Cut small slits atop each hand pie. (Alternately, use a smaller cookie cutter to cut out a shape from each top crust.)
Beat together the egg with 1 tablespoon water then brush the tops of the hand pies with the eggwash. Sprinkle the sanding sugar on top (optional) then bake the hand pies for about 15 minutes or until they're golden brown and the blueberry juice is bubbling.
Remove the hand pies from the oven and allow them to cool completely on the baking sheets before serving. (See Kelly's Note.)