Rinse whole lettuce leaves and pat dry, being careful not to tear. Set aside. Heat cooking oil in a large skillet over medium heat, add onion, cook (while stirring) until tender, about 5 to 10 minutes. Stir in hoisin sauce, garlic, soy sauce, vinegar, ginger and Asian chili pepper sauce. Shred chicken if in chunk form. Add water chestnuts, green onions, sesame oil and chicken. Cook and stir until the green onions just begin to wilt, about 2 minutes. Arrange lettuce leaves around the outer edge of a large serving platter and pile meat mixture in center or fully prepare and serve. Great warm or cold.