Sprinkle chicken with pepper. In a large skillet or wok, stir-fry chicken in 1 tablespoon oil until no longer pink. Remove and set aside. Stir-fry onion and peppers in remaining oil for 5 minutes. Add the water chestnuts, mushrooms and garlic; stir-fry 2-3 minutes longer or until vegetables are crisp-tender. Add stir-fry sauce and soy sauce. Stir in chicken; heat through. Place 1/2 cup chicken mixture on each lettuce leaf; sprinkle each with 1-1/2 teaspoons peanuts and 1/4 teaspoon cilantro. Fold lettuce over filling.