To make the ginger scallion oil: In a heatproof bowl, combine the scallions, ginger, and salt. Heat the cooking oil until very hot, then carefully pour it over the mixture and stir well.
To prepare the chicken: Clean the chicken and remove any remaining pin feathers.
To tighten the skin: Bring the water to a boil in a large stockpot. Carefully dip the chicken into the boiling water three times to tighten the skin.
To set the skin: Transfer the chicken to the ice water to cool and firm the skin.
To warm the cavity: Bring the poaching water back to a boil and add the salt. Dip the chicken into the hot water three more times, allowing the water to flow in and out of the cavity to warm the inside.
To poach the chicken: Fully submerge the chicken, reduce the heat to low, and add the scallions and ginger. Gently poach for about 45 minutes, or until the innermost thigh and thickest part of the breast reach 165°F (74°C) without touching the bone.
To serve: Carefully remove the chicken and chop, slice, or shred as desired. For firmer skin, briefly dip it in ice water again if desired. Serve with the ginger scallion oil.