Combine soy sauce, sesame oil, corn oil, onion, brown sugar, garlic, Chinese five spice, crushed red pepper and salt in large bowl. Cut chicken into small cubes; add to sauce. Stir in mushrooms and water chestnuts.
Refrigerate for 30 minutes or until ready to cook. Heat large non-stick skillet on medium-high heat. Add half of the chicken mixture. Cook, stirring occasionally until chicken is lightly browned and internal temperature is at least 170º F. Repeat with remaining chicken.
Remove from heat. Top with green onions. Serve in a lettuce leaf with a drizzle of Sweet and Tangy Soy Sauce.
SWEET & TANGY SOY SAUCE:
Combine all ingredients and refrigerate until ready to serve!