01 - Rinse chicken thoroughly under cold water. Place in a large stockpot and cover with 12 cups cold water. Add halved onion, garlic cloves, ginger, 2 whole green onions, and daikon radish. Bring to boil over medium-high heat, skimming off any foam that rises to surface.
02 - Reduce heat to low, cover partially, and simmer for 1 hour until chicken is cooked through and broth has developed flavor.
03 - Carefully remove chicken from pot and set aside to cool slightly. Strain broth through fine mesh sieve, discarding solids. Return clear broth to pot.
04 - Shred chicken meat into bite-sized pieces, discarding skin and bones. Return shredded chicken to broth. Season with 1 to 1½ teaspoons salt and black pepper, adjusting to taste.
05 - Simmer gently for another 10 minutes to meld flavors.
06 - In small mixing bowl, combine soy sauce, rice vinegar, sesame oil, sesame seeds, sliced green onion, and chili flakes.
07 - Serve hot soup in bowls, topped with remaining sliced green onion. Offer dipping sauce on the side.