Slice the flank steak into thin strips against the grain to ensure tenderness.

Place the sliced beef in a bowl and toss with cornstarch until each piece is evenly coated.

Prepare the sauce by combining soy sauce, water, brown sugar, minced garlic, grated ginger, and red pepper flakes in a bowl; stir until sugar dissolves.

Heat vegetable oil in a large skillet or wok over medium-high heat until shimmering.

Add the coated beef strips in a single layer, cooking in batches if necessary to avoid overcrowding, and sear until browned and crispy on the edges, about 2-3 minutes per side.

Remove the beef from the skillet and set aside.

Pour the prepared sauce into the hot skillet and bring to a simmer; cook for 2-3 minutes until it thickens slightly.

Return the beef to the skillet and toss to coat thoroughly with the sauce.

Cook for an additional 1-2 minutes to heat the beef through and allow the sauce to cling.

Remove from heat and garnish with sliced green onions and toasted sesame seeds before serving.