In a bowl, combine teriyaki sauce, sweet chili sauce, soy sauce, ginger, and red pepper flakes. Mix well and set aside.
In a large bowl, whisk together chicken broth, soy sauce, cornstarch, and 2 tablespoons oil until smooth. Add the sliced chicken and toss to coat. Let it sit for 15-20 minutes.
Bring a large pot of water to a boil. Drain off excess marinade, then add the chicken in batches and cook for 2-3 minutes, just until the chicken turns white. Drain well then coarsely chop the chicken into smaller pieces.
Drain well then coarsely chop the chicken into smaller pieces.
In the same skillet, heat the remaining 2 tablespoons of oil over medium-high heat. Add garlic, water chestnuts, and mushrooms. Sauté for 1 minute, until just tender.
Add velveted chicken and about half of the teriyaki mixture to the skillet. Cook on m medium-high heat for 4 minutes, stirring often until everything is heated through and well coated.
Stir in cashews and green onion and cook for 1 minute.
Place 1/4 cup of the chicken mixture in the center of each lettuce leaf, roll up, and enjoy!