Slice the flank steak into thin strips and coat evenly with cornstarch. Set aside for about 10 minutes.
Prepare the Mongolian Sauce by mixing soy sauce, brown sugar, water, garlic, and ginger together in a bowl.
Heat oil in a frying pan over medium-high heat and cook beef strips in batches until browned. Remove from pan.
Add the sauce mixture to the same pan and bring to a gentle simmer until it thickens, stirring continuously.
Return the beef to the pan, toss to coat with the sauce, and add green onions. Serve warm with rice or vegetables.
Notes
Always store leftovers in airtight containers to keep flavors fresh, and enjoy the meal within three days. Use fresh ginger and garlic for the best taste.