Slice the beef thin: Put the flank steak in the freezer for 15-20 minutes to firm it up. Find the direction the muscle fibers run, then slice across them, 1/4 inch thick. Cut any long strips in half so nothing is bigger than about 3 inches.

Coat in cornstarch: Toss the beef with the 1/3 cup cornstarch in a bowl or a zip-top bag until every piece is dusted. Shake off the extra. Loose cornstarch dumped into the skillet turns the sauce cloudy and pasty.

Mix the sauce: Whisk the soy sauce, brown sugar, water, sesame oil, and red pepper flakes together in a measuring cup until the sugar dissolves. Keep it next to the stove, because things move fast once the beef starts cooking.

Sear the beef in batches: Heat the vegetable oil in a large skillet or wok over high heat until it shimmers. Add half the beef in a single layer and let it sit undisturbed for 1-2 minutes, until the underside browns, then flip and cook 1 minute more. Transfer to a plate and repeat with the rest. Flank is a whole cut of beef, and USDA guidance for whole cuts is 145°F followed by a 3-minute rest. Strips sliced this thin can't be probed accurately, so cook until no pink remains, then let them rest on the plate while you build the sauce. They pass 145°F in the pan and go well past it again in the simmering glaze.

Cook the aromatics and sauce: Turn the heat to medium. Add the ginger and garlic to the empty skillet and stir for 30 seconds, just until they smell sweet. Pour in the sauce, scrape up the browned bits, and simmer for 3-4 minutes.

Thicken the glaze: Stir the 2 teaspoons cornstarch into the 1 tablespoon cold water, then whisk it into the simmering sauce. Let it bubble for 30-60 seconds, until it thickens enough to coat a spoon.

Toss and serve: Return the beef and any juices to the pan, add the green onions, and toss for 1-2 minutes until everything is coated and hot. Serve over the cooked white rice.