01 - Rinse the chicken thoroughly under cold water. Place in a large stockpot and cover with 12 cups (3 quarts) cold water.
02 - Add the halved onion, garlic cloves, sliced ginger, 2 whole green onions, and daikon radish chunks. Bring to a boil over medium-high heat, skimming off any foam that rises to the surface.
03 - Reduce heat to low, cover partially, and maintain a gentle simmer for 1 hour until the chicken is cooked through and broth is fragrant.
04 - Carefully remove the chicken from the pot and set aside to cool slightly. Strain the broth through a fine mesh sieve, discarding the cooked vegetables and aromatics. Return the clear broth to the pot.
05 - Shred the cooled chicken meat into bite-sized pieces, discarding skin and bones. Return the shredded chicken to the broth. Season with 1-1.5 teaspoons salt and black pepper, adjusting to taste.
06 - Simmer the soup gently for another 10 minutes to allow flavors to meld together.
07 - In a small mixing bowl, whisk together soy sauce, rice vinegar, sesame oil, toasted sesame seeds, finely sliced green onion, and chili flakes until well combined.
08 - Ladle hot soup into serving bowls and top with remaining sliced green onions. Serve immediately with the dipping sauce on the side.