In a large pot, combine the beef brisket with enough water to cover it completely. Bring to a vigorous boil, then reduce to low and simmer gently for about 30 minutes.

Skim any impurities or foam from the surface using a fine-mesh skimmer or spoon.

Stir in the soy sauce, gochujang, gochugaru, and minced garlic into the pot. Mix thoroughly and continue to simmer for about 5 minutes.

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

In a small pan, heat the sesame oil over medium heat. Add the sliced green onions and sauté for 2-3 minutes until fragrant.

Taste the soup and season with salt and pepper according to your preference, then serve hot with bowls of steamed rice.