Cut the flank steak into very thin strips against the grain, place it in a zip-top bag with the cornstarch, and shake until every piece is lightly coated. Each strip should look dusted rather than heavily breaded, with no thick clumps of starch.

Whisk the soy sauce, brown sugar, water, cornstarch, and red pepper flakes in a small bowl until smooth. Keep stirring until the sugar and cornstarch are fully dispersed and the mixture looks even.

Heat a large skillet over medium-high heat, add 2-3 tablespoons coconut oil, then sear half the beef in an even layer for 4-5 minutes before turning it and cooking for 2-3 minutes more; transfer it to a plate and repeat with the remaining oil and beef. The first side should develop pale golden, crisp-looking edges, and the beef should sizzle steadily rather than releasing a puddle of moisture.

Reduce the heat to medium, add more oil if needed, cook the green onions, garlic, and ginger for 1-2 minutes, then add the sauce and simmer it for 4-5 minutes before returning the beef to the skillet and serving immediately. The sauce is ready when its bubbles become slower and glossier and it coats the back of a spoon in a thin, shiny layer.