Combine the green parts of the spring onion with Szechuan peppercorns (reserve 5 peppercorns for later use) on a chopping board. Finely chop the spring onions and peppercorns together. Set aside.
Blanch the chicken in boiling water, drain and place back into the pot. Add enough water to completely cover the chicken. Add the ginger and reserved peppercorns and cook over high heat for 15-18 minutes or until the chicken juice runs clear. Remove from the pot and let cool. Reserve 2 tablespoons chicken stock.
Once the chicken has cooled, cut into 5cm x 1/2cm thin slices. Arrange the chicken and sliced lettuce on a serving dish.
In a small bowl, mix together the soy sauce, chicken stock, sesame oil and salt. Add the spring onion and peppercorn mixture. Pour over the chicken and lettuce, toss and serve.