In a large skillet, brown roast in butter on all sides. Transfer to a 5-qt. slow cooker; sprinkle with soup mix. Add cranberry sauce to skillet, stirring to loosen browned bits from pan. Pour over roast.
Cover and cook on low for 4-5 hours or until meat is tender. Remove roast to a serving platter; let stand for 10 minutes before slicing. Stir mustard into cooking juices. Serve with roast.