Instructions:
In a bowl, combine the thinly sliced flank steak with soy sauce, cornstarch, minced garlic, and grated ginger. Mix well to coat the beef evenly, then set aside to marinate for about 15-20 minutes.
While the beef is marinating, prepare the stir-fry sauce by combining soy sauce, hoisin sauce, brown sugar, rice vinegar, and sesame oil in a small bowl. Stir until the sugar is dissolved and set the sauce aside.
Heat 2 tablespoons of vegetable oil in a large skillet or wok over medium-high heat. Once hot, add the marinated beef slices in a single layer and cook for 2-3 minutes per side until browned and cooked through. Remove the beef from the skillet and set it aside on a plate.
In the same skillet, add another 2 tablespoons of vegetable oil. Add the sliced bell peppers and onion, and stir-fry for 3-4 minutes until they are tender-crisp.
Return the cooked beef to the skillet, then pour the prepared stir-fry sauce over the beef and vegetables. Stir well to coat everything evenly in the sauce and cook for an additional 1-2 minutes until heated through.
To serve, divide the cooked ramen noodles among serving bowls. Ladle the Mongolian beef stir-fry over the noodles, garnish with chopped green onions and sesame seeds, and serve hot.
