Soak and rinse the ribs. Place short ribs in a large bowl and cover with cold water for 20 to 30 minutes.
This draws out excess blood and helps keep the broth clear. Drain and rinse well.
Parboil for a clean start. Add ribs to a pot, cover with fresh cold water, and bring to a rolling boil for 5 minutes. Skim foam.
Drain and rinse the ribs under warm water. Rinse the pot, too. This step is key for a clear broth.
Build the base. Return the ribs to the clean pot.
Add 10 to 12 cups of water, onion halves, garlic, ginger, peppercorns, and the white parts of 2 scallions. Bring to a boil, then reduce to a gentle simmer.
Simmer low and slow. Cook uncovered at a gentle simmer for 1.5 to 2 hours. Skim any fat and foam as it rises.
Keep the simmer steady but not vigorous to maintain clarity.
Add the radish. After about 1 hour of simmering, add the radish chunks. Continue simmering until the ribs are very tender and the radish is easily pierced, about 30 to 45 minutes more.
Strain aromatics. Remove onion, ginger, garlic, and peppercorns. Keep the ribs and radish in the pot.
Taste the broth.
Season thoughtfully. Add salt in small amounts until it tastes round and savory. You can add a splash of soy sauce for depth, but keep the broth light and clear. A pinch of white pepper is nice, too.
Optional noodles. If using glass noodles, soak them in warm water until pliable.
Add to the hot broth for 2 to 3 minutes just before serving. Don’t overcook.
Finish and serve. Ladle the soup into bowls with a few pieces of rib and radish. Top with sliced scallion greens and a tiny drizzle of sesame oil if you like.
Serve with steamed rice and kimchi.