In a medium bowl, combine the sliced flank steak, soy sauce, and cornstarch. Mix well and let marinate for 15 minutes.
In a large skillet or wok, heat 1 tablespoon of vegetable oil 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 broccoli, garlic, and ginger, and stir-fry for 2-3 minutes until the broccoli is bright green and tender-crisp.
Return the beef to the skillet and add the cooked ramen noodles, beef broth, oyster sauce, sesame oil, and red pepper flakes if using. Toss everything together and cook for another 2-3 minutes until heated through.
Serve hot, garnished with sliced green onions.