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To make the crust: Weigh your flour, or measure it by gently spooning it into a cup, then sweeping off any excess. In a large bowl, whisk together the flour, sugar, salt, cardamom, and zest.
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Add the butter to the flour mixture, and work it in roughly with your fingers, a pastry blender, or a mixer. Don't be too thorough; the mixture should be quite uneven, with big chunks of butter in among the smaller ones.
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Drizzle 7 tablespoons (99g) of the ice water over the flour mixture, tossing gently to combine. Once the water is fully incorporated, squeeze some of the mixture in your hand. If it holds together easily without crumbling, it's ready. If it has dry spots or pieces break off, add the remaining 1 tablespoon water and gently mix until the dough comes together but is still slightly shaggy. Beware of kneading the pastry too much and/or adding too much water, as this will toughen the crust.
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Transfer the mixture to a piece of parchment paper. Press it into a rough rectangle and fold the dough into thirds, like a business letter. If necessary, spritz any dry areas with cold water and flatten and fold again, repeating the process until all errant bits of dough have been incorporated.
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Divide the dough into two pieces, one about 2/3 of the dough, and the other the remaining 1/3. Gather each piece into a rough rectangle about 3/4" thick. Smooth the surface and edges of the rectangles with a bench knife or bowl scraper.
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Wrap the pastry in plastic or your favorite reusable wrap. Chill for at least 30 minutes, or up to 2 days. (The pie dough can also be wrapped in aluminum foil and kept in the freezer for longer storage.) If dough has been chilled for longer than 30 minutes, allow it to warm at room temperature for 10 to 15 minutes before rolling.
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Preheat the oven to 425°F with a rack in the center.
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To make the filling: In a large bowl, whisk together the sugar, ClearJel or flour, cardamon, cinnamon, and salt. Add the fruit, zest, lemon juice, and vanilla, tossing to combine. Set aside.
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To assemble the slab pie: Line a baking sheet with parchment and set aside. On a well-floured work surface, roll the smaller piece of chilled dough into a rectangle measuring roughly 12" x 14" and between 1/16" and 1/8" thick.
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Use a knife or pizza wheel to slice the dough into 12 strips measuring 1" wide x 14" long. Carefully transfer the strips to the prepared baking sheet and refrigerate, uncovered, while you roll out the remaining dough.
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On the same well-floured work surface, roll the larger piece of dough into a rectangle measuring roughly 15" x 18" and about 1/8" thick.
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Transfer the dough to an ungreased 9" x 13" pan that’s at least 2" deep. (If dough feels delicate, fold it into quarters for an easier transfer.) Gently press the dough squarely into the corners.
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Spoon the filling into the crust and spread it into an even layer.
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Remove the dough strips from the refrigerator and arrange them on top of the filling in a lattice pattern, trimming them to fit as necessary. Roll the remaining dough around the edges down toward the filling to make a neat rim around the whole pie.
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Brush the top crust with the egg wash and sprinkle with sparkling sugar.
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Bake the pie for 50 to 70 minutes, tenting loosely with aluminum foil if the crust seems to be browning too quickly. When done, the filling will be bubbling and the crust deep golden brown.
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Remove the pie from the oven and let it cool at room temperature for at least 3 hours or up to 24 hours before slicing and serving.
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Storage information: Store leftover slab pie, well-wrapped, in the fridge for 5 days, or freeze for up to 3 months.
