For the crust: Make both batches of shortcrust pastry separately according to the recipe instructions. Wrap each batch into plastic wrap and refrigerate for 30 mintues.
Take the pastry out of the refrigerator. Unwrap one batch and place on a lightly floured surface. Roll out to a large circle, then fit into a lightly greased 9-inch (23-cm) pie plate, pressing it down gently so it eases into the bottom of the plate. Trim the excess dough, leaving a 1-inch (2.5-cm) overhang all around. Refrigerate.
Unwrap the second batch of shortcrust pastry and transfer to a lightly floured work surface. Roll out the pastry to a rough rectangular shape about 1/8-inch (0.3-cm) thick. Using a sharp knife (and a ruler to help, if desired), cut about 20 strips of dough that are 1/2-inch (1.25-cm) wide. Line a baking sheet with parchment paper. Arrange 10 of the dough strips horizontally 1/2-inch (1.25-cm) apart. These will be the “bottom” strips.
Fold back every other bottom strip to the left slightly more than halfway. Slightly left of center, lay down one “top” strip vertically over the bottom strips. Unfold the bottom strips. Fold back the other 5 bottom strips to the left, and lay a second strip 1/2-inch (1.25-cm) to the right of the first. These are the two center strips. Unfold the bottom strips. Repeat the process on both sides with the remaining strips of dough. (See note for instructions to make a more elaborate lattice pattern.)
Loosely cover the lattice with plastic wrap and place the baking sheet in the freezer while you prepare the filling.
For the wild blueberry filling: Preheat oven to 400°F (200°C).
Place the wild blueberries in a large bowl. Scoop up about 1 cup (250 ml) of blueberries and put them in another medium-sized bowl. Add the sugar, cornstarch, flour, lemon zest and juice, and the salt. Mash with a potato masher or a fork until the cornstarch and flour have dissolved and the mixture is almost homogeneous (there will be bits of blueberries throughout).
Pour the mixture in the large bowl containing the remaining blueberries and carefully fold it in until the blueberries are all coated with the “glue”. Transfer the mixture into the chilled pie shell, gently pressing down on the berries if needed so that they all fit into the pie (they will cook down a little).
To assemble: Remove the lattice from the freezer, and carefully invert it over the blueberry filling. The lattice should be pretty stiff by now, so let it rest about 5 minutes so it softens a little. Trim excess lattice dough, leaving a 1/2-inch overhand all around. Tuck edge of lattice between the edge of the bottom dough and the rim of the pan. Using your fingers, press both layers of dough along the edge to seal, and crimp as desired. Place the pie on a parchment-lined baking sheet.
In a small bowl, whisk together the egg yolk and water. Brush the lattice with the egg mixture, then sprinkle with coarse sugar all over. Bake until the crust begins to turn golden, about 20 minutes. Reduce the oven temperature to 350°F (175°C). Continue baking, rotating the pie halfway through, until the crust is deep golden brown and juices are bubbling and thick, 40 to 50 minutes more.
Transfer the lattice-top blueberry pie to a wire rack to cool for at least 2 hours before serving it. The pie is best eaten the day it is baked, but you can make it up to 1 day ahead and store it, covered loosely with plastic wrap when cool, at room temperature. Leftovers keep in the refrigerator for up to 5 days.