To make the pie dough: In a food processor, combine the flour, granulated sugar, and salt and pulse a few times until blended. Add the cold butter pieces and pulse until the mixture resembles coarse meal, about 10 pulses.
Add 8 Tbsp of the ice water and pulse 2 or 3 times. The dough should hold together when squeezed with your fingers but should not be sticky. If it is crumbly, add more water 1 Tbsp at a time, pulsing twice after each addition.
As soon as the dough holds together, turn it out onto a work surface, using your hands to bring it together. There should be visible striations of butter.
Divide the dough in half and shape each into a rectangle. Wrap the rectangles separately in plastic wrap and refrigerate for at least 2 hours or up to overnight.
For the filling: In a large bowl, toss together the blueberries, granulated sugar, cornstarch, lemon zest, vanilla and salt. Let stand while you roll out the dough.
Let the dough stand at room temperature for 30 minutes. On a lightly floured work surface, roll out 1 dough rectangle into a 22-by-18-inch (55-by-45-cm) rectangle about 1/8 inch (3 mm) thick.
Roll the dough around the rolling pin and unroll it on top of a baking sheet. Gently press the dough into the bottom and sides of the pan. Trim the edges with scissors or a knife, leaving a 1/2-inch (12-mm) overhang. Refrigerate while rolling out the dough for the lattice crust.
When ready to cook, start the Traeger according to grill instructions. Set the temperature to 400 degrees F and preheat, lid closed, for 10 to 15 minutes.
Roll out the remaining dough rectangle into a 22-by-18-inch (55-by-45-cm) rectangle. Use a ruler to mark six 1-inch (2.5-cm) spaces along the short sides of the dough.
With a fluted pastry roller, a pizza cutter or a knife, cut six 1-by-22-inch (2.5-by-55-cm) strips from the dough. Gather up the scraps and reroll the dough into an 18-inch (45-cm) square. Again, mark 1-inch (2.5-cm) spaces along the dough and cut six 1-by-18-inch (2.5-by-45-cm) strips from the dough.
You will use only 5 of each size strips for the lattice, but you’ll have an extra in case 1 breaks. If the dough becomes too warm at any point, refrigerate it before continuing.
To make the lattice crust, pour the blueberry filling into the chilled crust and spread it out evenly. Place 5 of the longer strips lengthwise across the pie, spacing them equal distance apart.
Fold back every other strip halfway and lay down a short strip perpendicular across the unfolded strips. Repeat to place a total of 5 strips of dough evenly across the top, folding back the alternate strips each time.
Trim any overhang of lattice strips. Pinch the lattice strips onto the dough on the rim of the baking sheet to make sure they bake together.
Brush the lattice crust with beaten egg mixture and sprinkle with the coarse sugar. If the dough is very warm, refrigerate for 10 minutes before baking.
Place the slab pie on the lowest shelf of the grill, close the lid, and bake until the crust is golden and the filling is bubbling, about 45 minutes, tenting the pie with aluminum foil if the top is browning too quickly.
Transfer the baking sheet to a wire rack and let the pie cool for at least 1 hour before slicing and serving. Enjoy!
Note: The pie can be baked up to 1 day ahead; let cool completely, then cover with foil and let stand at room temperature.
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