In a steaming pot or a deep pan with a rack, place a large flat-bottom plate on the rack. Put a small bowl upside down in the center of the plate.
Lay the chicken legs against the upside-down bowl. Place two slices of ginger on each chicken leg.
Bring the water to a boil, then steam the chicken over medium-high heat for 25–30 minutes. ***To check doneness, poke the thickest part of the leg with a chopstick: if the liquid runs clear, the chicken is fully cooked.
Carefully remove the chicken legs and set them aside, or transfer them to a water bath until warm to the touch. Then shred the meat off the bone into bite-size pieces.
Once the plate has cooled slightly, lift the bowl to release the collected chicken stock underneath. Save it for the sauce.
In a heatproof bowl, add the chopped scallion and garlic. Heat the oil until smoking hot, then carefully pour it over the scallion and garlic. You should hear an immediate sizzle.
Add soy sauce, salt, sugar, chicken bouillon and 4-6 tbsp of reserved chicken stock. Mix together.
Pour the scallion sauce generously over the steamed chicken. Serve as-is, or with steamed rice and some blanched greens.