Combine the beef brisket with enough water to cover it in a large pot. Bring to a boil and then reduce heat to simmer for 30 minutes, keeping the lid slightly ajar.

Skim off any impurities that have risen to the surface to achieve a clear broth.

Incorporate soy sauce, gochujang, gochugaru, and minced garlic into the broth. Stir thoroughly and simmer for another 5 minutes.

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

Heat sesame oil in a small pan. Sauté sliced green onions for 2-3 minutes, add to the pot.

Season with salt and pepper to taste, adjusting as needed.

Ladle soup into bowls and serve hot with white rice.