Freeze the flank steak for 20–30 minutes until slightly firm to make slicing easier. Slice thinly (1/8–1/4 inch) across the grain. Place the slices in a bowl and toss with 3 tbs cornstarch until evenly coated; set on a tray to rest for 5–10 minutes.

Prep aromatics

Peel and mince 1 tbs ginger and 3 cloves garlic. Slice 4 scallions on a bias, keeping white/light-green parts separate from the dark green tops. Break or crush 6 dried red chiles (or measure red pepper flakes) and set aside.

Heat the wok and fry beef

Heat a wok or large skillet over high heat until very hot. Add 2 tbs vegetable oil and swirl. Working in a single layer and in batches if necessary, add coated beef slices in a single layer and sear without crowding for 60–90 seconds per side until browned and slightly crisp around edges. Remove browned beef to a plate and repeat with remaining beef, adding more oil as needed (total about 3 tbs used).

Sauce

Mix the sauce

While frying beef or beforehand, whisk together 1 cup low-sodium soy sauce, 1 cup packed brown sugar, 1 cup water, 1 tbs rice vinegar, 1 tsp hoisin, 1 tsp chili garlic sauce (or Sriracha), and 1 tsp dark soy (optional). Stir until sugar is mostly dissolved.

Reduce sauce in wok

After removing the last batch of beef, wipe the wok if there are excessive burnt bits but keep remaining fond. Add remaining 1 tbs vegetable oil if the wok is dry, then add minced ginger, minced garlic and the white/light-green parts of the scallions plus the dried chiles. Stir-fry 20–30 seconds until fragrant, taking care not to burn the garlic.

Combine beef and sauce

Pour the prepared sauce into the wok and bring to a vigorous simmer over high heat. Let it boil for 3–5 minutes, stirring occasionally, until it thickens and becomes glossy (the sugar will caramelize into a sticky glaze). If sauce thickens too quickly, add a splash of water. Return the seared beef to the wok and toss constantly for 1–2 minutes so each piece becomes well coated and heated through.

Finishing / Optional

Finish and serve

Turn off the heat, stir in 1 tsp toasted sesame oil and sprinkle 1 tsp toasted sesame seeds. Toss in the dark green scallion tops and 2 tbs chopped cilantro or extra scallion greens. Taste and adjust heat with extra chili garlic sauce or crushed red pepper if desired.

Serve immediately over steamed rice or fried rice, and enjoy. Leftovers keep in the refrigerator for up to 3 days.