Begin by soaking the rice noodles in warm water for about 20-30 minutes or until softened. Drain and set aside.
In a bowl, mix together the soy sauce, brown sugar, hoisin sauce, and sesame oil. Set this sauce mixture aside.
Heat a large skillet or wok over medium-high heat and add the vegetable oil.
Once hot, add the sliced flank steak in batches to avoid overcrowding. Sear for about 2-3 minutes until browned and cooked through. Remove the beef from the skillet and set aside.
In the same skillet, add the minced garlic and grated ginger. Stir-fry for about 30 seconds until fragrant.
If using, toss in the sliced bell pepper and cook for another 1-2 minutes until tender.
Return the cooked beef to the skillet and pour in the prepared sauce mixture. Cook for an additional 2-3 minutes, stirring to evenly coat the beef and bell peppers.
Add the drained rice noodles to the skillet, tossing to combine everything and heat through for about 2 minutes.
Season with salt and pepper to taste, then remove from heat.
Serve hot, garnished with sliced green onions and chopped fresh cilantro on top.