To prepare broth, add water to large pot, followed by kombu, stiitake mushrooms, green onions, and red pepper flakes. Stir ingredients together and bring to a boil.
Once boiling, remove kombu and discard. Add napa cabbage to broth and blanch cabbage in broth for 1-2 minutes, then remove to a bowl. Reduce heat to medium-low and allow broth to simmer for about 10 minutes.
For the sauce, add garlic, doenjang, gochugaru, sesame oil, fish sauce, mirin, and black pepper to a small bowl. Mix well with a spoon.
Place beef chuck in another bowl and top with half the sauce. Stir to coat well and set aside.
Tear blanched cabbage into strips and add to a bowl. Top with remaining sauce and stir to coat. Set aside.
Strain vegetables from broth before assembling soup.
Heat a large pot over high heat and add marinated beef chuck and radish to hot pot. Cook 1-2 minutes, then add broth and napa cabbage mixture. Stir and bring to a boil. Once boiling, reduce heat to medium and simmer for 15-20 minutes.
Add green onions and soybean sprouts. Continue to simmer for 5-10 minutes, then remove from heat.
Enjoy with warm bowl of rice, kimchi, and some banchan (Korean side dishes). Use chopped green onion for garnish if desired.