In a bowl, combine soy sauce, brown sugar, and rice vinegar.
Add the sliced flank steak to the marinade and let it sit for 10 minutes.
Sprinkle cornstarch over the marinated beef and toss to coat evenly.
Heat vegetable oil in a large skillet over medium-high heat.
Add the beef to the skillet and stir-fry until browned, about 3-4 minutes.
Remove the beef from the skillet and set aside.
In the same skillet, add garlic, ginger, and bell peppers.
Stir-fry the vegetables until tender, about 2-3 minutes.
Return the beef to the skillet and pour in any remaining marinade.
Cook for an additional 2 minutes, stirring to combine.