In a bowl, combine the sliced flank steak, soy sauce, and cornstarch. Mix well and let marinate for 15 minutes.
Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the marinated steak and stir-fry for about 3-4 minutes until browned. Remove the steak from the skillet and set aside.
In the same skillet, add the remaining tablespoon of vegetable oil. Add the sliced bell peppers, broccoli, and snap peas. Stir-fry for 4-5 minutes until the vegetables are tender-crisp.
Add the minced garlic and ginger to the skillet, cooking for an additional minute until fragrant.
Return the cooked steak to the skillet. Pour in the beef broth and oyster sauce, stirring to combine. Cook for another 2-3 minutes until everything is heated through.
Drizzle with sesame oil before serving. Serve hot over cooked rice or noodles.