Prepare the Beef: Thinly slice the flank steak against the grain for maximum tenderness. Toss the beef slices with cornstarch until evenly coated. Let it sit for 5 minutes to absorb the cornstarch.
Prepare the Sauce: In a small bowl, whisk together the soy sauce, brown sugar, beef broth, hoisin sauce, sesame oil, and red pepper flakes if using. Set aside.
Sear the Beef: Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add half the beef in a single layer and sear for 1-2 minutes on each side until browned and crispy. Remove the beef and set aside. Repeat with the remaining beef, adding more oil if necessary.
Sauté Garlic and Ginger: In the same skillet, add the remaining tablespoon of oil. Add the minced garlic and grated ginger. Sauté for 30 seconds until fragrant.
Add the Sauce: Pour the prepared sauce into the skillet and bring to a simmer. Cook for 2-3 minutes, allowing the sauce to thicken.
Cook the Ramen Noodles: While the sauce is simmering, cook the ramen noodles in boiling water for 2-3 minutes until just tender. Drain and set aside.
Combine Beef, Noodles, and Sauce: Add the seared beef back to the skillet along with the cooked ramen noodles. Toss everything together, ensuring the noodles and beef are evenly coated with the sauce.
Garnish and Serve: Sprinkle sliced green onions and sesame seeds over the top before serving. Enjoy hot!