Instructions

1. Make the dough: In a large bowl, combine the flour and salt. Add 1 1⁄2 cups lukewarm water and the egg whites and stir with fingers. (Dough should be shaggy with dry pockets of flour, like biscuit dough.)
2. On a well-floured work surface using floured hands, knead the dough, dusting with more flour as needed, until smooth, about 4 minutes. Transfer to a lightly floured bowl and cover the bowl with plastic wrap. Let rest 30 minutes or up to 1 hour.
3. Make the filling: In a medium skillet, heat 2 tablespoons oil over medium-high heat. Add the onion and cook, stirring occasionally, until softened, 7–9 minutes. Stir in the chile oil. Let cool.
4. In a large bowl, combine the beef, scallions, oyster sauce, soy sauce, ginger, sesame oil, pepper, and salt and mix with hands. Fold in the cooled onions.
5. On a floured surface, knead the dough briefly until satin smooth. Cut into 4 equal pieces. Roll each piece into a ¾-inch-thick log and cut with a cleaver or a sharp knife into 12 equal pieces about the size of an egg yolk. On a rimmed baking sheet, toss the balls generously with flour and drape with a damp paper towel to hold.
6. Flatten each ball slightly with the palm of your hand. Using an Asian-style rolling pin, flatten the dough a bit more. Roll from the edge of each dough disk to its center, rotating the disk between rolls. Repeat until the wrapper is 3 inches in diameter and the edges are half as thick as the center. Transfer back to the well-floured work surface and tent with a damp paper towel. Repeat with the remaining wrappers.
7. Holding a wrapper in your palm, fill the center with a tablespoon of filling. Pinch the edges of the wrapper shut to form a half moon, squeezing out any air bubbles. Tug the ends of the dumpling slightly to elongate, then curve into a slight crescent shape. Repeat with the remaining filling and wrappers (you may have a few extra wrappers).
8. In a medium bowl, whisk the flour, vinegar, and 2 cups water to make a slurry.
9. Brush one or two 10-inch nonstick or cast-iron skillets lightly with oil and heat over medium-high heat. Add 8 dumplings to each pan with the flat sides down, at least 1⁄2 inch apart. Pour in cup plus 1 tablespoon of the slurry, then partially cover the pan. Increase to high heat and cook 1 minute for nonstick or 2 minutes for cast iron. Lower the heat to medium and cook 3 minutes for nonstick or 2 minutes for cast iron. Lower the heat to low, and cook 3 minutes for nonstick or 2 minutes for cast iron. Remove the lid and cook until water has evaporated and a golden brown starch disk remains, 3–4 minutes more. Using a flexible spatula, loosen the disk from the pan. Place a large plate over the top of the pan and flip the disk onto the plate in one motion, crispy side up. Serve immediately.