1. Slice the flank steak thinly against the grain and toss it with the cornstarch until evenly coated. Let it rest for 10 minutes.

2. In a small saucepan, combine the soy sauce, water, and brown sugar. Heat over medium heat, stirring until the sugar dissolves and the mixture begins to simmer. Set aside.

3. Heat the vegetable oil in a large skillet or wok over medium-high heat. Cook the beef in batches for 2 to 3 minutes per batch until browned and lightly crisp around the edges. Transfer to a plate.

4. Drain most of the oil from the skillet, leaving about 1 tablespoon behind. Add the ginger and garlic, then cook for 30 seconds until fragrant.

5. Pour the prepared sauce into the skillet and simmer for 2 to 3 minutes until slightly thickened.

6. Return the beef to the skillet and toss to coat well in the sauce.

7. Add the green onions and cook for 1 to 2 minutes more until just softened.

8. Serve hot over cooked white rice and garnish with sesame seeds if desired.