1tablespooncold unsalted butter, cut into small pieces
Fold 1 dough round in half and carefully transfer to a 9-inch pie dish. Unfold and ease the round into the pan, without stretching it, and pat it firmly into the bottom and up the sides of the pan. Using kitchen scissors, trim the edge of the dough, leaving 3/4 inch of overhang. Set the dough-lined pan aside, along with the other dough round, in a cool place until ready to use.
Place the blueberries in a large bowl, sprinkle with the lemon juice and toss to coat evenly. In a small bowl, stir together the sugar, cornstarch, lemon zest, salt and cinnamon. Sprinkle the sugar mixture over the berries and toss to distribute evenly. Immediately transfer to the dough-lined pan. Dot with the butter.
Roll out the second pie dough disk into a 12-inch round. With pastry wheel, cut the dough into 10 strips 3/4 inch wide. Place 5 strips horizontally across the top of the pie at 1 inch intervals.
Weave in 5 vertical strips and trim and flute the edges decoratively with your thumb.
Refrigerate the pie until the dough is firm, 20 to 30 minutes. Meanwhile, position a rack in the lower third of an oven and preheat to 375°F.
Before baking, brush the crust with an egg wash. To make the egg wash, lightly beat an egg, add a tablespoon of milk to thin it, and brush onto the crust with a pastry brush. Sprinkle crust with granulated sugar. To keep your pie crust from burning or browning too quickly, wrap aluminium foil along the edge of the crust and cook 25 minutes. Take off aluminum foil after 25 minutes and continue to bake.
Bake the pie until the crust is golden and the filling is thick and bubbling, 50 to 60 minutes. Transfer to a wire rack and let cool completely to set, 1 to 2 hours. Serve at room temperature or rewarm in a 350°F oven for 10 to 15 minutes just before serving.