Stack lettuce leaves on top of each, place sauce on top and cut to make perfectly round leaves. In small bowl, stir in cornstarch with the sherry. Set aside. In frying pan over medium, warm 2 tablespoons of oil. Add onion and stir fry until soft. Add ginger and cook for 1 minute. Add celery and mushrooms and saute for 5 minutes. Add chicken and saute for 3 minutes. Reduce heat to low and add cornstarch mixture. Cook for 1 minute. Add soy sauce and bamboo shoots and fry until heated through. Transfer to warmed baking dish and serve lettuce leaves alongside. The guests take a lettuce leaf and roll up the meat mixture in the leaf.