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1
Make the pastry. Combine the water, rice vinegar, and ice in a cup. Combine the flour, sugar, and salt in a food processor. Pulse to combine. Add the butter and sesame oil. Pulse until the butter is broken up-some walnut-sized pieces, some blueberry-sized, some pea-sized. Err on the side of under- versus over-processing. This is key to a flaky crust! Dump this floury, buttery mixture into a big bowl. Add a couple tablespoons of the vinegary ice water and toss with your hands. Add more, if needed, to create a sort of dry, sort of shaggy dough. Stick in the fridge or freezer to let rest for 10 or so minutes, giving the flour a chance to absorb the water. Toss again and add more water if necessary to create a dough that holds together when pressed. Form into a rough rectangle and wrap tightly in plastic. Refrigerate for at least 1 hour or up to 2 days.
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2
When you're ready to make the crostatas, preheat the oven to 400u00b0 F. Line a rimmed baking sheet with a silicone mat or parchment.
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3
Combine the soy sauce, sesame oil, brown sugar, and pepper flakes in a large bowl. Stir until the sugar dissolves. Add the scallion and toss.
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4
Divide the pie pastry into four equal pieces. Working on a barely floured surface, roll each into a rectangle roughly 6 by 5 inches. Place these rectangles onto the baking sheet, evenly spaced out. Divide the scallion mixture evenly between the rectangles and spread out, leaving a 1-inch or so border. Fold this border over the scallion mixture to create a crust. Brush the crust with the egg wash.
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5
Bake for about 25 minutes, rotating halfway through, until the crust is crisp and colorful. Serve warm or at room temperature. These are best the day of.
