Cook the rice noodles according to package instructions. Drain and set aside.
In a bowl, combine soy sauce, brown sugar, and cornstarch. Mix well and set aside.
Heat vegetable oil in a large skillet or wok over medium-high heat. Add the sliced flank steak and season with salt and pepper. Stir-fry for about 3-4 minutes until browned. Remove the beef from the skillet and set aside.
In the same skillet, add garlic and ginger. Sauté for about 30 seconds until fragrant.
Add the red bell pepper and snap peas to the skillet. Stir-fry for 2-3 minutes until the vegetables are tender-crisp.
Return the beef to the skillet and pour the soy sauce mixture over the top. Stir well to combine and cook for an additional 2 minutes until the sauce thickens.
Add the cooked rice noodles and sesame oil to the skillet. Toss everything together until the noodles are well coated and heated through.
Serve hot, garnished with chopped green onions and sesame seeds if desired.