In a large pot, place the beef brisket and add enough water to cover it. Bring to a rolling boil, then reduce heat to low and simmer for about 30 minutes, or until tender.

While simmering, skim off any impurities or foam that rises to the surface using a ladle. This helps achieve a clear broth.

Add soy sauce, gochujang, gochugaru, and minced garlic once the beef is tender, stir to dissolve gochujang, and simmer for additional 5 minutes.

Add chopped onion, napa cabbage, firm tofu, and daikon radish to the pot. Cover and let simmer for another 20 minutes.

In a skillet, heat sesame oil over medium heat. Add sliced green onions and sauté for about 2 minutes. Stir into soup.

Taste and adjust seasoning with salt and pepper. Simmer for an additional 5 minutes if needed.

Ladle the soup into bowls and serve hot with cooked white rice and extra green onions if desired.