Place flank steak in the freezer for 15 minutes to firm up, then slice thinly against the grain into strips.
Toss the beef slices evenly in cornstarch and shake off excess.
Heat vegetable oil in a large skillet or wok over medium-high heat.
Fry the coated beef in batches for 2–3 minutes per side until golden and crispy. Transfer to a paper-towel-lined plate.
Drain excess oil from the pan, leaving about 1 tablespoon.
Sauté minced garlic and grated ginger in the pan for 30 seconds until fragrant.
Add soy sauce, water, and brown sugar to the pan. Stir to combine and bring to a simmer.
Let the sauce cook for 3–4 minutes until slightly thickened.
Return the crispy beef to the pan and toss to coat evenly in the sauce.
Remove from heat, drizzle with sesame oil, and add green onion pieces.
Serve immediately over steamed jasmine rice or white rice
