Start by slicing the flank steak into bite-sized strips. Chop the carrots into matchsticks, slice the bell peppers, and mushrooms. Mince the garlic and grate the ginger. Set all your ingredients aside.
Bring a large pot of water to a boil. Add the brown rice noodles and cook according to package instructions until al dente. Drain the noodles in a colander and rinse them under cold water to stop the cooking process. Set aside.
In a bowl, combine the soy sauce, brown sugar or honey, gochujang, rice vinegar, grated ginger, minced garlic, and toasted sesame oil. Mix well until the sugar is dissolved and set aside.
Heat a wok or large skillet over medium-high heat. Add 1 tablespoon of sesame seed oil and let it heat up. Add the sliced flank steak in a single layer and cook for about 2-3 minutes, or until browned. Remove the beef from the skillet and set aside.
In the same skillet, add the remaining tablespoon of sesame seed oil. Add the carrots, bell peppers, and mushrooms. Stir-fry for about 3-4 minutes, or until the vegetables are tender but still crisp.
Return the cooked beef to the skillet with the vegetables. Pour in the sauce and stir well to combine everything. Cook for another 2-3 minutes, allowing the sauce to thicken slightly.
Add the cooked noodles to the skillet and toss everything together until the noodles are coated with the sauce and combined with the beef and vegetables.
Remove the skillet from the heat. Serve the stir-fried beef and noodles in bowls, garnished with chopped green onions, sesame seeds, and fresh cilantro. Enjoy your delicious 30-Minute Stir Fried Korean Beef and Toasted Sesame Noodles!
