Heat 3-4 quarts water to a boil in a large pot over medium-high heat. Add the brisket, onion, and shiitake mushrooms. Reduce heat to low, the cover and simmer 1 hour, or until beef is tender. (You may need longer, up to 2 hours).
Skim the fat from the top, then remove the brisket, and mushrooms. Set aside. Discard the onion.
Strain the broth and return it to the pot.
In a large bowl, combine the garlic, bean sprouts, pre-soaked fernbrake, and scallions. Stir in the gochugaru, gochujang, sesame oil, soy sauce, and salt and pepper to taste.
Stir the vegetable mixture into the pot and simmer for 15 minutes.
Slice the reserved beef (against the grain, or you can shred it) and mushrooms (if desired). Return to the pot and simmer for 5-10 minutes to warm. Adjust for salt and pepper to taste.
Serve hot in bowls with cooked glass noodles (if desired).
