Combine diced chicken with cornstarch. Refrigerate while you prepare the rest of the ingredients.
Combine all sauce ingredients and set aside.
Heat olive oil to a pan over medium heat. Add ginger, garlic and the whites of the green onions and cook until fragrant, about 2 minutes.
Add chicken and cook until almost cooked through, about 3-4 minutes. Add water chestnuts, bell pepper and sauce. Simmer 2-3 minutes or until chicken is cooked through.
To serve, place a scoop of the warm chicken filling in a lettuce leaf. Add desired toppings and drizzle with a little bit of hoisin sauce.
(Nutrition information provided is an estimate and will vary based on cooking methods and brands of ingredients used.)