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 cook for about 3-4 minutes until browned. Remove the beef from the skillet and set aside.
In the same skillet, add the minced garlic, bell pepper, and snap peas. Stir-fry for 2-3 minutes until the vegetables are tender-crisp.
Return the beef to the skillet and add the hoisin sauce, soy sauce, sesame oil, and red pepper flakes (if using). Stir well to combine and heat through for another 2 minutes.
Add the cooked noodles to the skillet and toss everything together until the noodles are well coated with the sauce.
Serve hot, garnished with sliced green onions and sesame seeds.