Start by cooking the rice noodles according to the package instructions. Once cooked, drain them well and set aside to prevent sticking.
In a medium bowl, combine the thinly sliced flank steak with the cornstarch. Toss until the beef is evenly coated.
Heat the vegetable oil in a large skillet or wok over high heat. When the oil is shimmering, carefully add the beef in a single layer. Sear for 2-3 minutes without moving it, then flip and cook for an additional 2 minutes. Transfer the beef to a plate and set aside.
In the same skillet, reduce the heat slightly, and add the sliced onion, minced garlic, and grated ginger. Stir-fry for 1-2 minutes until fragrant.
Add the sliced bell pepper to the skillet. Stir-fry for another 2-3 minutes until the bell pepper is tender but still slightly crisp.
Return the cooked beef to the skillet along with the chopped green onions. Pour in the soy sauce, sprinkle the brown sugar, drizzle the sesame oil, and add the red pepper flakes if desired. Stir everything together.
Gently add the cooked rice noodles to the skillet, tossing everything together in the sauce. Stir-fry for an additional 2-3 minutes.
Taste the dish and season with salt and pepper as needed. Adjust sweetness or saltiness by adding more brown sugar or soy sauce.
Notes
Present in large bowls and garnish with green onions and sesame seeds.