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.
When you’re ready to make the crostatas, preheat the oven to 400° F. Line a rimmed baking sheet with a silicone mat or parchment.
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.
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.
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.
Emma is the food editor at Food52. Before this, she worked a lot of odd jobs, all at the same time. Think: stir-frying noodles "on the fly," baking dozens of pastries at 3 a.m., reviewing restaurants, and writing articles about everything from how to use leftover mashed potatoes to the history of pies in North Carolina. Now she lives in Maplewood, New Jersey with her husband and their cat, Butter. Stay tuned every Tuesday for Emma's cooking column, Big Little Recipes, all about big flavor and little ingredient lists. And see what she's up to on Instagram at @emmalaperruque.