In a medium bowl, whisk together soy sauce, brown sugar, garlic, ginger, 2 teaspoons vegetable oil and 1/2 cup water. Heat soy sauce mixture in a medium saucepan until slightly thickened, about 5-10 minutes; set aside.
In a large bowl, combine flank steak and cornstarch.
Heat 1/2 cup vegetable oil in a large saucepan. Add beef and fry until browned and cooked through, about 1-2 minutes. Transfer to a paper towel-lined plate; discard excess oil.
Add beef and soy sauce mixture to the saucepan over medium heat and cook until sauce thickens, about 2-3 minutes. Stir in green onions.
1 pound flank steak, thinly sliced across the grain
1/4 cup cornstarch
1/2 cup vegetable oil
2 green onions, thinly sliced
For the sauce
1/4 cup reduced sodium soy sauce
1/2 cup brown sugar, packed
3 cloves garlic, minced
2 teaspoons grated fresh ginger
2 teaspoons vegetable oil