For the dough: Combine the ice and water in a small cup or bowl. Set aside.
Put the flour, sugar, and salt in a large bowl and stir with a spatula to combine.
Sprinkle in the butter cubes and toss until each cube is coated in the flour mixture.
Flatten each cube of butter with your pointer fingers and thumbs. Toss again to coat the flattened butter pieces. Continue massaging the butter into the flour until the remaining shards resemble cornflakes in a range of sizes.
Add 6 tablespoons of the cold water, taking care not to include any ice, and fluff the moisture through the flour with a spatula. Continue adding cold water 1 tablespoon at a time, pressing the dough with a spatula after each addition until it begins to come together. Avoid any heavy kneading, as overworking the dough will lead to a tough crust.
If the dough still has quite a bit of dry mix and doesn’t hold together when a handful is squeezed, add a little more water.
When the dough begins to hold together, turn it out onto your work surface and gently form it into a rounded mound with your hands. Cut the mound in half. Wrap each half of dough tightly in plastic, then gently press into a round, flat disk, about 5 inches in diameter and 1 inch in thickness. Refrigerate for at least 3 hours or overnight before rolling. If you plan to freeze the pie dough, do so only after the rest period of at least 3 hours in the fridge.
For the spoke pie: Roll 1 dough disk into an 11 × 15-inch rectangle. Roll the dough onto the rolling pin and unfurl it onto a sheet of parchment paper. Using a ruler as a straight edge and a rolling pastry wheel, cut the rectangle widthwise into at least 30¼-inch strips.
Slide a flat baking sheet under the parchment and place the dough in the refrigerator to keep cold while proceeding with the pie.
Roll the second dough disk into a 14-inch circle. Roll the dough onto the rolling pin and unfurl it over a 9-inch pie pan. Taking the edges of the dough, gently ease the dough into the pan, nestling it into the inner elbows of the pie pan. Trim the excess dough with kitchen shears to create a 1-inch overhang. Fold the overhang back under, creating an elevated edge.
For the bluebarb filling: Set aside ¼ cup (43 grams) of the blueberries. Combine the remaining blueberries, rhubarb, sugar, tapioca starch, lime juice, and salt in a large bowl. Gently fold with a spatula until all the fruit pieces are coated.
Pour the filling into the pie shell.
Assemble the pie: Place a 2-inch circle cutter in the center of the filling as a reference point. Using a small pastry brush, lightly dab water around the edge of the pie shell.
Remove the dough strips from the fridge. Gently pick up a strip of dough, handling it only from the ends, and lay it across the pie, with the strip grazing the outside of the circle cutter. Lightly press the strip into the edges of the pie to secure.
Place another dough strip across the pie. The center of this second strip should lay slightly on top of the first and also graze the center circle cutter. The end of the strip in your left hand should rest ½ inch to the left of the first strip and the end in your right hand to the right of the first.
Continue laying dough strips in this fashion, working your way around the pie surface twice. Resist the urge to manually curve each strip. The optical illusion of the full design inspires a curved effect, but the individual strips should be maintained as straight lines.
Press the edges of the pie to secure the strips in place. Holding a paring knife at a 45-degree angle to your work surface, run it around the edge of the pie pan to trim the excess dough.
Gently remove the center ring cutter and carefully fill the crater with the ¼ cup of reserved blueberries.
Chill the entire pie in the freezer until the oven has come to temperature. The pie can be frozen solid or simply chilled through, about 20 minutes.
Preheat the oven to 425°F. Line a rimmed baking sheet with parchment and prepare the egg wash.
When the oven has reached temperature, remove the pie from the freezer and place it on the prepared baking sheet. Brush with the egg wash and sprinkle with demerara sugar, if desired.
Bake the pie for 25 minutes, then rotate the pie 180 degrees and lower the oven temperature to 350°F. If the edges are already brown, cover with a shield. If the top begins to brown excessively, rest a sheet of foil lightly on top. Continue baking until the filling is bubbling in the center, checking every 30 minutes to rotate the pie and adjust the shields as necessary, 80 to 100 total minutes. (If baking from frozen, add 30 to 45 minutes to the bake time.)
Cool the pie completely on a rack before slicing and serving.
Egg washing note: An egg wash can add an extra dimension of deep golden shine that is hard to resist.
Milk: brown matte finish
Egg white: clear and shiny finish One whole egg: yellow and shiny finish
One whole egg with a splash of heavy cream: deep golden shiny finish
