Soak the rice noodles in warm water for 30 minutes, then drain.
In a large pot, heat vegetable oil over medium-high heat.
Add the sliced beef to the pot and sauté for 2-3 minutes until browned.
Stir in the sliced onion, garlic, and ginger, and cook for an additional 3-4 minutes.
Pour in the beef broth, soy sauce, gochujang, and brown sugar. Stir well.
Bring the mixture to a simmer, then reduce heat to low and cover for 15 minutes.
Add the soaked noodles to the broth and cook for 5-7 minutes until soft.
Taste and adjust seasoning if necessary, adding more soy sauce or gochujang as desired.
Serve hot, garnished with chopped green onions and sesame seeds.