In a large pot, add the cold water, scallions, and ginger, and bring to a boil, and boil for 2 minutes. Add the chicken cutlet, bring to a boil, and boil for 2 minutes. Turn off heat, cover, and allow to cool to room temperature.
Remove the chicken from pot, place on a chopping board, and hit smartly with the broad side of cleaver blade. This will break the fabric of the meat. Then with your hands pull the meat apart, shredding it.
Place the shredded chicken in the bowl with the sauce and mix together well. Add the sliced scallions and mix well again. Reserve.
Dry-roast the sesame seeds (see below). When done, allow to cool, then pour into bowl with the chicken and sauce. Mix well together. Prepare a serving dish by making a border of the shredded lettuce. Then pour the chicken into the center of the dish and serve immediately.
TO DRY ROAST
Heat the wok over high heat for 30 to 45 seconds. Add the seeds and lower the heat to very low. Spread the seeds in a single layer and use a spatula to move them about and turn them over to avoid burning on one side. This process takes about 2 to 3 minutes, or until the seeds turn brown. Turn off the heat, remove the seeds from the wok, and allow them to cool. Seeds can be dry-roasted two to three days in advance of their use. After they cool, place them in a sealed jar.