Cook the rice noodles according to package instructions. Drain and set aside.
In a large skillet or wok, heat the vegetable oil over medium-high heat. Add the sliced flank steak and stir-fry for about 3-4 minutes until browned. Remove the beef from the skillet and set aside.
In the same skillet, add the bell pepper, snap peas, garlic, and ginger. Stir-fry for 2-3 minutes until the vegetables are tender-crisp.
In a small bowl, whisk together the soy sauce, brown sugar, rice vinegar, sesame oil, and red pepper flakes. Pour this sauce over the vegetables in the skillet and stir to combine.
Return the beef to the skillet and add the cooked rice noodles. Toss everything together until well coated and heated through, about 2-3 minutes.
Serve immediately, garnished with chopped green onions and sesame seeds.