Begin by thinly slicing the flank steak against the grain. In a mixing bowl, combine the beef with soy sauce and cornstarch, ensuring every piece is coated. Allow it to marinate for about 15 minutes.

Heat 2 tablespoons of vegetable oil in a large skillet or wok over medium-high heat until shimmering. Carefully add the marinated beef to the hot oil and stir-fry for approximately 3-4 minutes, or until the beef is browned and no longer pink. Remove the beef from the skillet and set it aside.

In the same skillet, add a little more oil if needed and toss in the sliced onion, bell pepper, and minced garlic. Stir-fry for about 3-5 minutes until the vegetables are tender yet still crisp.

Add the freshly minced ginger into the skillet with the sautéed vegetables. Stir continuously for about 1 minute until the ginger releases its fragrant aroma.

Return the cooked beef to the skillet, then pour in the beef broth and oyster sauce, mixing thoroughly. Cook for an additional 2-3 minutes, allowing the flavors to meld.

Drizzle the stir fry with sesame oil, season with salt and pepper to taste, and sprinkle green onions on top before serving.