In a large pot, add the beef brisket and water. Bring to a boil over high heat, then reduce to a simmer for about 30 minutes until the beef is tender. Keep the lid slightly ajar to prevent overflows and stir occasionally.

Skim off any impurities that rise to the surface using a fine-mesh skimmer. Gently skim the top every few minutes until the broth is clear and free of foam.

Add the soy sauce, gochujang, gochugaru, and minced garlic into the pot. Stir well to combine, making sure the chili paste dissolves fully into the broth for an even, rich color.

Add the chopped onion, napa cabbage, tofu cubes, and daikon radish. Continue to simmer the soup for another 20 minutes, allowing the vegetables to soften and the tofu to absorb the spicy, savory flavors.

In a small pan, heat the sesame oil over medium heat. Add the sliced green onions and saute until fragrant, about 1-2 minutes. Pour this aromatic oil and onion mixture into the soup.

Season the soup with salt and pepper to taste. Adjust as necessary - add more soy sauce for depth or a pinch of gochugaru for extra heat.

Serve hot with steamed white rice on the side. Ladle generously into bowls, letting the rice soak up the spicy broth for a satisfying, warming meal.