Begin by cooking the rice noodles according to package instructions. Drain and set aside.
In a medium bowl, toss the sliced flank steak with cornstarch until well coated.
Heat vegetable oil in a large skillet or wok over high heat. Once hot, add the beef in a single layer. Sear for 2-3 minutes until browned, then flip and cook for another 2 minutes. Remove from pan and set aside.
In the same skillet, add the sliced onion, garlic, and grated ginger. Stir-fry for 1-2 minutes until fragrant and the onion begins to soften.
Add the sliced bell pepper and continue to stir-fry for another 2-3 minutes until vegetables are tender-crisp.
Return the cooked beef to the skillet and add the chopped green onions. Drizzle with soy sauce, brown sugar, sesame oil, and red pepper flakes (if using). Stir to combine all ingredients evenly.
Gently fold in the cooked rice noodles and toss everything in the sauce for about 2-3 minutes until well mixed and heated through.
Season with salt and pepper to taste. Adjust sweetness or saltiness if needed by adding more sugar or soy sauce.
Notes
Serve in large bowls, garnished with green onions and sesame seeds.