1Arrange chicken, thickest parts to outside edge, in 2-cup microwavable casserole or on microwavable plate. Cover tightly and microwave on High 3 to 4 minutes or until juice is no longer pink when centers of thickest pieces are cut. Uncover and refrigerate about 10 minutes or until cool enough to handle.
2Cut chicken into 1/4-inch pieces. Toss chicken, dressing, mustard, cucumber, tomato and cashews. Cover and refrigerate about 1 hour or until chilled.
3Just before serving, spread cut sides of croissant halves with butter. Spread chicken mixture over bottoms of croissants; top with lettuce and tops of croissants.