Combine teriyaki sauce, chili sauce, soy sauce, ginger, and red pepper flakes in a bowl.
Mix well and set aside.
In a large skillet, heat oil over medium heat. When oil is hot, add chicken.
Season chicken lightly with salt and pepper. When chicken is white on all sides (doesn't need to be completely cooked through), remove chicken from skillet and set aside on a plate. Discard any extra juices.
In the same skillet, heat 2 Tbsp oil over medium-high heat. Add onion, garlic, water chestnuts, pinenuts, and mushrooms and saute for about 1-2 minutes.
Stir in about 1/2 of the teriyaki mixture and chicken to the skillet. Cook uncovered, for about 7-8 minutes, stirring often until chicken is cooked through. Serve with butter lettuce.