Prep the lettuce: Begin by cutting off the end of the lettuce, then slowly peel off each individual leaf. Wash each leaf of lettuce, pad them dry with some paper towels and then put them on a plate and in your fridge to keep cool.
Brown up the chicken: Add one tablespoon of oil to a wok, or skillet and place over high heat. Then add the chicken and cook until it has browned up about 5 minutes. Take the chicken out and set aside.
Saute the veggies: Add another tablespoon of oil to the wok/skillet and add in the onion and bell pepper. Saute these for 5 minutes. Then add in the garlic and shiitake mushrooms and saute everything for another 2 minutes.
Finish the dish: Add back the chicken along with the water chestnuts and the sauce ingredients. Stir everything together for 2 to 3 minutes until the sauce has reduced and covered everything.
Serve the stir-fried ingredients with the lettuce leaves on a plater and let everyone make their own lettuce wraps. Enjoy!