Heat a large skillet or wok with a generous amount of cooking spray. Chop chicken breasts into thin strips (or cubes depending on what you prefer), and place in warm pan along with the onions. Pour 2 tbs Teriyaki sauce, 1 tbs soy sauce, and garlic powder onto the chicken. If you like, add additional seasonings to the chopped chicken (I added some pepper, and 1 packet of stevia). Stir until chicken is evenly coated with sauce.
Cook the chicken for about 4-6 minutes or so, turning it frequently for even cooking. (Note: During this step I usually add about 1/4 to 1/2 cup water a little at a time to the pan to help the chicken cook through, and keep it moist…Also the time will vary on how long to cook the chicken, but I suggest ‘eyeballing it’ and cooking it until the chicken is lightly browned on the outside, and there is no pink on the inside, onions should start to get translucent at this point). Take chicken and onions out of the pan when it is fully cooked, and set it aside.
Add the frozen vegetables, water chestnuts, and the rest of the Teriyaki sauce to the pan, (you may add a bit more seasoning here too if desired!) Cook veggies for about 3 to 5 minutes, or until completely de-thawed and cooked through. Turn off the heat.
Put the chicken back in the pan with the veggies, mixing until everything is well combined. Divide mixture into four even servings. Scoop the hot chicken and veggie stir fry onto lettuce wraps and garnish with sesame seeds if desired. Enjoy!