Preheat oven to 375°. Defrost souffles in microwave at 50% power for 6 to 7 minutes. In large bowl, combine spinach, rice, milk, half the cheese, onion, mustard, salt and pepper. Stir well. Place in a 9 x 13-inch baking dish. Cover and bake for 25 minutes. In large skillet, sauté chicken in oil until golden. Arrange chicken on top of spinach mixture. Mix well and sprinkle over chicken. Bake, uncovered, 25 minutes longer, or until spinach is set.