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

2. In a small bowl, whisk together the soy sauce, brown sugar, and water until smooth.

3. Heat the vegetable oil in a large skillet over medium-high heat.

4. Add the beef in batches and cook until browned and lightly crisp on the edges, about 2 to 3 minutes per batch. Transfer the cooked beef to a plate.

5. Carefully drain most of the oil from the skillet, leaving about 1 tablespoon behind.

6. Add the garlic and ginger to the skillet and cook for 30 seconds until fragrant.

7. Pour in the sauce mixture and simmer for 2 to 3 minutes until it thickens slightly.

8. Return the beef to the skillet and toss until fully coated in the glossy sauce.

9. Stir in most of the green onions and cook for 1 more minute.

10. Serve hot over cooked white rice and garnish with the remaining green onions and sesame seeds.