For the crust: Put the flour, yellow cornmeal, 2 tablespoons sugar and the salt in a food processor and pulse to combine. Add the butter and shortening and pulse until the mixture resembles coarse breadcrumbs. Combine 1/2 cup ice water and the apple cider vinegar in a liquid measuring cup. Add the ice water mixture to the flour mixture, 1 tablespoon at a time as needed, and pulse until the mixture just comes together. Turn the dough out onto a clean surface and press the dough together. Divide the dough in half and press into two discs. Refrigerate the dough until firm, at least 2 hours.
For the filling: Put 4 cups blueberries and 1/3 cup sugar in a medium saucepan over medium-high heat and cook, stirring frequently, until the sugar dissolves and the blueberries start to release some of their juices, 3 to 5 minutes. By this time, the mixture will have started to boil. Lower the heat to a strong simmer and cook, stirring occasionally, until the mixture reduces, 10 minutes. Stir in the tapioca starch and cook until the mixture thickens and resembles, blueberry jam, 5 minutes.
Transfer the mixture to a large bowl, then stir in the remaining 4 cups blueberries, the remaining 1/3 cup sugar, and the blueberry jam, lemon zest, lemon juice and salt. Cool completely, about 45 minutes.
To assemble and bake: Roll 1 chilled dough disc into a 13-inch round (about 1/8-inch-thick) on a lightly floured surface.
Roll the dough onto a rolling pin, then unroll it into a 9 1/2-inch glass deep-dish pie plate, letting it hang over the edge. Evenly spoon the chilled filling into the pie plate.
Roll out the remaining dough disc into a 12-inch round and place it over the filling. Press the crust edges together and trim, leaving a 1/2-inch overhang. Roll up or pinch the overhang to seal. Crimp as desired.
Make five 2-inch slashes in the top of the crust. Chill for 30 minutes.
Position a rack on the lowest shelf of the oven and place a baking sheet on the rack. Preheat the oven to 425 degrees F.
Whisk the egg and 1 tablespoon water in a small bowl to make an egg wash. Brush the egg wash on top of the chilled pie, including the crimped edges. Sprinkle with the remaining 1 tablespoon sugar.
Put the pie on top of the baking sheet in the oven and bake for 20 minutes. Lower the temperature to 375 degrees F and rotate the baking sheet. Bake, rotating the pan halfway through for even color, until the crust is golden brown and the filling is bubbling, about 1 hour. If the edges begin to brown too quickly, cover them with foil or crust protectors. Remove the baking sheet from the oven and allow the pie to cool completely on the baking sheet.
Slice the cooled pie into 8 to 10 pieces before serving.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)