In a bowl, toss the sliced flank steak with cornstarch until evenly coated. Let it sit for about 10 minutes.
Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat.
Add the beef in a single layer and cook for about 2-3 minutes until browned. Remove the beef from the skillet and set aside.
In the same skillet, add the remaining tablespoon of oil. Sauté the white parts of the green onions and minced garlic for about 30 seconds until fragrant.
In a bowl, mix together soy sauce, brown sugar, water, rice vinegar, and ginger. Pour this sauce into the skillet and bring to a simmer.
Return the beef to the skillet and cook for another 2-3 minutes, stirring to coat the beef in the sauce until heated through.
Serve the Mongolian beef over cooked white rice and garnish with the green parts of the green onions.
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