For the pastry: In a large bowl, combine the flour, sugar and salt. Using a pastry blender, cut the butter into the flour mixture until you have something that resembles coarse meal with a few larger pea-sized pieces of butter. Add 6 tablespoons ice water and stir with a fork. If you can squeeze the mixture together in your hands, you have added enough water; if not, add a little more. The dough will look shaggy. Knead the dough with your hands a couple times in the bowl to get it to start to come together. Tip the dough out onto a piece of plastic wrap. Use the edges of the plastic wrap to help gather the dough and form it into a flat rectangle. Refrigerate for at least 1 hour.
For the fillings: In a large saucepan, combine the blueberries, 1/2 cup of the sugar, 3 tablespoons of the cornstarch and the salt. Cook over medium heat, stirring occasionally, until the blueberries have released some juice and the sauce is thick, 6 to 8 minutes. Let cool completely.
In a medium bowl, beat together the cream cheese, lemon zest and the remaining 1 tablespoon cornstarch and 1/4 cup sugar until smooth. Beat in the egg yolk. Preheat the oven to 400 degrees F.
For the egg wash: In a small bowl, whisk together the egg yolk with 1 tablespoon water.
On a lightly floured piece of parchment, roll the dough out into a rectangle about 12 by 14 inches. Trim the ragged edges. (Pop the dough in the fridge to chill if it has gotten too soft.) Spread the cream cheese filling on the right half of the dough rectangle, leaving a 1 1/2-inch border around the edges. Top with the cooled blueberry filling. Brush the border with the egg wash. Use the parchment to lift the left half of the dough over the filling. Use a fork to seal the edges. Using the parchment, transfer the pie to a baking sheet. Brush the top with more egg wash and sprinkle with the sanding sugar if using. Use a small paring knife to cut a few small slits in the top to vent the pie. Bake until golden brown and set, 40 to 50 minutes. Transfer to a rack to cool before slicing.
Recipe courtesy of Samantha Seneviratne