In a small bowl, mix together the soy sauce, sherry, sugar, and cornstarch.
In a large skillet or a wok heat two tablespoons of the oil over high heat, add half the ginger stir for 1 minute until you can smell the ginger, then add the half the scallions and the chicken and sauté for about 3 minutes until the chicken is mostly white on the outside but still slightly raw inside and the scallions have started to soften. Remove the chicken and scallions from the skillet to a serving bowl and set aside.
Repeat with the remaining oil, ginger, scallions and chicken and when the chicken is mostly cooked out the outside, return the cooked chicken and scallions to the pan, along with any juices that have accumulated, and add the soy sauce mixture. Stir to coat the chicken with the mixture. Add the chicken broth and sesame oil, bring to a simmer, and allow the sauce to thicken while the chicken finishes cooking, about 3 minutes more.
Serve hot, sprinkled with the sesame seeds if desired.