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Blueberry Sumac Slab Pie

yield: 1 9 x 13-inch pie
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Special Equipment

  • A food processor
  • a quarter sheet pan (or other 9 x 13-inch pan with shallow sides)
  • a sharp paring knife

Ingredients

For the Pie Dough

  • 1/2 cup ice
  • 1/2 cup very cold water
  • a dash of apple cider vinegar
  • 2 1/2 cups (12.5 ounces) all-purpose flour, plus more for rolling dough
  • 1 tablespoon granulated sugar
  • 1 teaspoon kosher salt
  • 1 cup (2 sticks // 8 ounces) very cold unsalted butter, cut into rough 1-inch cubes

For the Blueberry Sumac Filling

  • 1/3 cup (2.35 ounces) granulated sugar
  • 1 tablespoon sumac (see baker's notes for sources)
  • 2 cups (12 ounces) fresh blueberries
  • 1 cup (5 ounces) fresh cherries, stemmed, pitted, and halved
  • 1/4 cup (1 ounce) cornstarch
  • a pinch of kosher salt

For the Egg Wash and Topping

  • 1 large egg
  • 1 teaspoon water
  • demarara (or other coarse) sugar, for sprinkling

Instructions

For the Pie Dough

  • In a large liquid measuring cup, combine 1/2 cup ice, 1/2 cup very cold water, and a dash of apple cider vinegar. Give it a good whisk with a small whisk or fork, and transfer to the refrigerator to keep chilled while you work with the dry ingredients.
  • In a food processor, pulse together 2 1/2 cups flour, 1 tablespoon granulated sugar, and 1 teaspoon kosher salt several times to combine. Add 1 cup very cold unsalted butter cubes. Pulse until the mixture resembles coarse meal with pea-sized pieces remaining.
  • Sprinkle with 4 tablespoons water from the ice water mixture in the 1st step. Pulse until dough is crumbly but holds together when squeezed with fingers; if necessary, add up to 2 tablespoons more water, 1 tablespoon at a time. Be careful not to overprocess or add too much liquid! Pulse until it just holds together.
  • Turn out the dough onto a lightly floured work surface. Divide dough into two even halves. Form each half into a disk. Wrap tightly in plastic, and refrigerate until firm, at least 1 hour, preferably overnight.

For the Blueberry Sumac Filling

  • In a small bowl, combine 1/3 cup granulated sugar and 1 tablespoon sumac. Use your fingers to rub the sumac into the sugar, to help infuse the sumac flavor into the sugar. Transfer to a medium bowl.
  • Add 2 cups fresh blueberries, 1 cup fresh cherries, 1/4 cup cornstarch, and a pinch of kosher salt. Toss to combine — the mixture will be slightly goopy, but that's what you want.

Assembly

  • First, make the base crust. Remove one of the pie dough disks from the refrigerator and use a rolling pin to roll into a rectangle slightly larger than a quarter sheet pan — if you're using a 9 x 13-inch pan, aim for a 11 x 15-inch rectangle. Transfer to your sheet pan and use your fingers to press the crust firmly up against the sides of the pan, trimming any excess overhang.
  • Next, use a rubber spatula to scoop the blueberry sumac filling evenly over the rolled out base crust and transfer to the refrigerator as you make the leaf crust topping. Remove the second pie dough disk and roll into a rough circle about 1/4-inch thick. Use a paring knife to carve out as many leaf shapes as you can, gathering and re-rolling any scraps as necessary. Use the paring knife to score the center and edges of the leaf shapes to create leaf texture. If the dough is getting too soft and hard to work with, transfer to the freezer to chill for 10 to 15 minutes. Lay the leaves in a random pattern across the pie filling, cover loosely with plastic wrap, and transfer to the freezer to chill for 1 hour, but preferably overnight for best results.
  • When you’re ready to bake the pie, center a rack in the oven and preheat to 350 (F). Quickly make an egg wash by whisking together 1 large egg and 1 teaspoon of water in a small bowl; use a pastry brush to brush the egg wash over the leaves, and sprinkle immediately with demerara sugar.
  • Transfer the quarter sheet pan to the center rack of the oven and bake for around 60 minutes, or until the crust is golden brown and the fruit juices bubble. Transfer to a wire rack and cool for about an hour or so, to allow the juices to set. Serve slightly warm, with a scoop of vanilla ice cream or fresh whipped cream. Enjoy!