In a medium skillet, heat the 2 Tbsp vegetable oil over medium-high heat until it starts to shimmer. Add the onions and cook, stirring occasionally, for 9 to 10 minutes, until they turn soft and translucent. Stir in the chile oil, then remove the pan from the heat and set aside to cool.
In a medium bowl, use your hands to combine the beef, scallions, soy sauce, oyster sauce, ginger, sesame oil, pepper and salt, and mix until well blended. Gently fold in the onions and mix until fully incorporated.
Make the dumplings. Holding a wrapper in your palm, use a fork to add about 1 tablespoon of the filling to the center of the wrapper, then lightly pat down the filling with the fork to get rid of any air bubbles.
Fold the dumplings in a half-moon, pinching it shut with your thumbs and index fingers, then press the center of the dumplings while pulling on the corners to push out any air bubbles and shape into a curved crescent. Inspect the dumplings for any holes and pinch them shut. Repeat with the rest of the wrappers to make 24 dumplings.
In a small bowl, whisk together the flour, vinegar, and 1 cup of water until combined to make a slurry. Brush the remaining 1 teaspoon oil in a medium cast-iron or nonstick skillet and heat over medium-high heat, until the oil starts to shimmer. Add 6 dumplings with the sealed edges lying slat in the pan, spacing them 1 inch apart, then slowly pour in just enough of the slurry to come one-third of the way up the dumplings. Partially cover the pan, leaving a small gap for steam to escape.
Increase the heat to high and cook for 2 minutes for cast iron (1 minute for nonstick). Lower the heat to medium for 2 minutes for cast iron (3 minutes for nonstick). Then lower the heat to low for 2 to 3 minutes for cast iron (3 minutes for nonstick).
Cook until the water has evaporated, leaving a paper-thin disk of golden-brown starch on the bottom of the pan. Remove from heat and slide a thin, flexible spatula around the rim of the pan to loosen the edges of the starch disk, then carefully slide the spatula underneath and flip the disk onto a plate in one piece, crispy side up. Serve immediately, then clean the skillet and repeat its remaining dumplings. Serve immediately.