Make the broth: Add beef, water, onion, garlic, ginger, and scallion pieces to a large pot. Bring to a boil, then reduce to a gentle simmer.
Skim foam. Cook 1.5–2 hours, until beef is very tender.
Strain and shred: Remove beef and set aside to cool slightly. Strain the broth and discard aromatics.
Shred the beef into bite-size strands with forks.
Build the chili oil: In a clean pot, heat neutral oil over low. Add gochugaru and stir 30–45 seconds until fragrant and red. Do not burn.
Season the base: Stir in gochujang, soy sauce, fish sauce (if using), sesame oil, and sugar.
Cook 30 seconds to melt and bloom the flavors.
Add broth and simmer: Pour the strained beef broth into the pot. Bring to a simmer and taste. Season with salt and pepper as needed.
Add vegetables: Add shredded beef, bean sprouts, gosari, napa cabbage, and mushrooms.
Simmer 10–12 minutes, until vegetables are tender but not mushy.
Finish with noodles: Add soaked glass noodles and cook 4–6 minutes, until chewy and translucent.
Optional egg ribbons: Slowly drizzle in beaten eggs while gently stirring to create silky strands.
Garnish and serve: Ladle into bowls. Top with sliced scallions. Serve with hot steamed rice and kimchi.