Start by slicing the flank steak against the grain into thin strips, about 1/4 inch thick.
Toss the sliced beef with cornstarch until well-coated.
In a large skillet or wok, heat the vegetable oil over medium-high heat and add the coated beef strips in a single layer.
Cook the beef for about 2-3 minutes on each side until browned and crispy. Remove and set aside.
In the same skillet, sauté minced ginger and garlic for 30 seconds. Then add soy sauce, brown sugar, water, and red pepper flakes.
Once the sauce thickens slightly (about 2-3 minutes), return the beef to the skillet and toss until well-coated.
Cook for an additional minute, then remove from heat and garnish with sliced green onions.
