Pat the beef dry with paper towels. Slice thinly against the grain into 1/4-inch strips.
Toss the beef with cornstarch until lightly coated. Shake off excess and let it sit 10 minutes to adhere.
In a bowl, whisk soy sauce, water, and brown sugar until dissolved.
Stir in garlic, ginger, and red pepper flakes. Set aside.
Heat a large skillet or wok over medium-high until hot. Add 1–2 tablespoons oil.
Cook beef in batches without crowding. Spread in a single layer and sear 1–2 minutes per side until browned with crisp edges. Transfer to a plate and repeat with remaining beef, adding more oil if needed.
Reduce heat to medium. Pour the sauce into the pan. Simmer 2–3 minutes until glossy and slightly thick.
Return the beef (and any juices) to the pan. Toss to coat. Add green onions and cook 30–60 seconds.
Turn off the heat and stir in sesame oil. Taste and adjust heat or sweetness if needed.
Cold beef, hot pan: Keep sliced beef chilled until cooking to help crisp the edges.
Don’t overcoat: Light cornstarch coverage is enough—excess turns gummy.
Work in batches: Crowding steams the beef and kills the sear.
Heat control: Sear on high, simmer sauce on medium to prevent burning sugar.
