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 to the skillet, season with salt and pepper, 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, green onions, garlic, and ginger. Stir-fry for about 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 rice noodles to the skillet, tossing everything together until the noodles are well coated with the sauce and heated through.
Serve immediately, garnished with additional green onions if desired.