In a large pot, place the whole chicken, sliced onion, peeled garlic cloves, halved green onion, and black peppercorns. Fill with enough water to submerge the chicken.

Cover the pot and bring the mixture to a boil over medium heat. Once boiling, reduce the heat and simmer for 25 minutes.

Carefully remove the chicken, shred the meat off the bones, and discard the skin and bones.

Return the bones to the pot and simmer the broth on low for another 30 minutes.

Strain the broth using a fine-mesh strainer into a clean pot, discarding the solids.

Mix flour, potato starch, and salt for the kalguksu. Gradually add water until a soft dough forms.

Knead the dough on a floured surface for 10-15 minutes until smooth and elastic. Let it rest covered for 1 hour.

Roll the dough to about 0.3 cm thickness and cut into strips for noodles.

Bring the strained broth back to a boil, add shredded chicken and noodles, cook for 3-4 minutes until tender.