Steps

1. 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.
2. Add 8 tablespoons 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 tablespoon at a time, pulsing twice after each addition.
3. 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.
4. Divide the dough in half and shape each half into a rectangle. Wrap the rectangles separately in plastic wrap and refrigerate for at least 2 hours or up to overnight. Let the dough stand at room temperature for 30 minutes before rolling out.
5. 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.
6. On a lightly floured work surface, roll out 1 dough rectangle into a 22 x 18-inch rectangle about 1/8 inch thick.
7. Wrap the dough around the rolling pin and unroll it on top of a rimmed 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 overhang. Refrigerate while rolling out the dough for the lattice crust.
8. Roll out the remaining dough rectangle into a 22 x 18-inch rectangle. Use a ruler to mark six 1-inch spaces along the short sides of the dough.
9. With a fluted pastry roller, a pizza cutter or a knife, cut six 1 x 22 inch strips from the dough. Gather up the scraps and re-roll the dough into an 18 inch square. Again, mark 1-inch spaces along the dough and cut six 1 x 18-inch strips from the dough.
10. 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.
11. To make the lattice crust, pour the blueberry filling into the chilled bottom crust and spread it out evenly. Place 5 of the longer strips lengthwise across the pie, spacing them equal distance apart.
12. 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.
13. 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.
14. Beat the egg with 1 teaspoon water. 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.
15. When ready to cook, set Traeger temperature to 400°F and preheat, lid closed for 15 minutes.
16. Place the slab pie directly on the grill grate, 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.
17. Transfer the baking sheet to a wire rack and let the pie cool for at least 1 hour before slicing and serving. Enjoy!