In a medium bowl, combine the sliced flank steak, soy sauce, and cornstarch. Mix well and let marinate for at least 15 minutes.
Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the marinated beef and stir-fry for about 3-4 minutes until browned. Remove the beef from the skillet and set aside.
In the same skillet, add the remaining tablespoon of vegetable oil. Add the bell pepper, broccoli, and snap peas. Stir-fry for about 3-5 minutes until the vegetables are tender-crisp.
Add the minced ginger and garlic to the skillet and stir-fry for an additional 1-2 minutes until fragrant.
Return the beef to the skillet. In a small bowl, mix together the beef broth, brown sugar, rice vinegar, and sesame oil. Pour this mixture over the beef and vegetables. Stir well to combine and cook for another 2-3 minutes until the sauce thickens slightly. Season with salt and pepper to taste.
Remove from heat and stir in the chopped green onions. Serve hot over cooked rice or noodles.