In a large mixing bowl, combine the sliced flank steak with cornstarch, ensuring each piece is evenly coated. Allow the beef to marinate for about 15 minutes.

Heat 2 tablespoons of vegetable oil in a skillet over medium-high heat. Add marinated beef in batches, cooking for 3-4 minutes until browned. Set aside.

In the same skillet, add minced garlic and sauté for about 30 seconds until fragrant. Do not burn.

Whisk together soy sauce, brown sugar, ginger, and sesame oil in a bowl. Pour into the skillet and bring to a gentle simmer.

Return sautéed beef to the skillet and toss to coat. Simmer for an additional 2-3 minutes.

Add chopped green onions, stirring to combine. Adjust seasoning with salt and pepper if necessary. Serve hot over rice or noodles.