With a sharp knife, make 3 or 4 parallel slashes in each piece of chicken. Rub with Creole Seasoning and salt, spreading seasonings into slashes. Place chicken, meat side down, in a 2-quart rectangular glass dish. Cover with wax paper and microwave on high 5 minutes. Turn chicken pieces over, re-cover and microwave on high. Stir onions into tomato mixture and pour over chicken; re-cover. Microwave on high 5 minutes or until an instant-read thermometer inserted in thickest part of breast registers 170ºF.