First Step: Prepare Beef and Broth
Soak the beef brisket in cold water for 20 minutes, changing the water several times to remove excess blood. In a large pot, combine 10 cups of water, soaked beef, halved onion, white parts of the green onions, and black peppercorns. Bring to a boil over high heat, skimming off any foam or impurities for 15 minutes. Reduce to medium-low, cover, and simmer for 45 minutes to develop a rich beef broth.

Second Step: Strain and Shred Meat
Remove the beef from broth and let cool. Strain the broth through a fine sieve, discarding solids. Trim excess fat from the beef and shred it finely into thin strips for texture and soup integration.

Third Step: Sauté Aromatics and Chili
Heat a clean pot over medium-low heat. Add cooking oil, sesame oil, and the white parts of green onions, sauté until softened and aromatic. Stir in the Korean chili flakes (gochugaru) and cook for about 30 seconds to infuse the oil, being careful not to burn the chili flakes. Remove the pot from heat temporarily if needed.

Fourth Step: Combine Broth and Ingredients
Add strained beef broth, shredded beef, and shiitake mushrooms back into the pot. Raise the heat to medium-high and bring to a boil. Stir in prepared fernbrake. If using, add chili oil, gochujang, and doenjang at this point. Season with Korean soup soy sauce, fish sauce, minced garlic, salt, and pepper. Reduce to medium heat, cover, and simmer for 30 minutes to blend flavors.

Fifth Step: Add Bean Sprouts and Scallions
Add bean sprouts and the green parts of the green onions. Cook uncovered for an additional 10 minutes to maintain freshness and slight crunch.

Sixth Step: Optional Ingredients
For added texture and substantial body, incorporate soaked sweet potato starch noodles a few minutes before finishing, allowing them to cook fully. Optionally, drizzle lightly beaten eggs into boiling soup just before finishing and turn off heat to create delicate egg ribbons.

Final Step: Adjust Seasoning and Serve
Taste soup and adjust with additional salt and pepper if needed. Garnish generously with thinly sliced green onions. Serve hot, traditionally alongside steamed rice, kimchi, and other Korean side dishes for an authentic and nourishing meal.