Prepare the Beef: Toss sliced flank steak in cornstarch until evenly coated. Let it rest for 10 minutes to help the coating stick.
Fry the Beef: Heat oil in a wok or large skillet over medium-high heat. Fry beef in small batches for 1–2 minutes per side until crisp. Transfer to a paper towel–lined plate.
Make the Sauce: In a separate pan, heat 2 tbsp oil. Add garlic and ginger; sauté until fragrant. Stir in soy sauce, water, and brown sugar. Simmer until slightly thickened, about 3–4 minutes.
Combine: Add fried beef to the sauce and toss until evenly coated. Stir in green onions and cook for 1 minute.
Serve: Serve hot with steamed rice and extra sauce spooned over the top.
Notes
Use flank steak or sirloin for best texture. Substitute coconut aminos for soy sauce for a gluten-free option.