Peel and cut potatoes lengthwise in half; cut crosswise into 1/2-in. slices. Place potatoes and apples in a 6-qt. slow cooker. In a small saucepan, mix butter, brown sugar and pie spice. Bring to a boil over medium heat; cook until blended, 1-2 minutes. Pour over potato mixture. Cook, covered, on low until potatoes are tender, 5-6 hours., With a slotted spoon, remove potatoes and apples to a bowl, reserving cooking liquid. Mash potato mixture, gradually adding enough reserved cooking liquid, if necessary, to reach desired consistency.