Use the transient

<?php
/*
 * This is how to access and build a nested areas of practice list.
 */
 $areas_of_practice = ssc_get_areas_list();

 foreach($areas_of_practice as $parent_area => $child_areas) {
  echo '<ul class="top-level"><li>';
  echo '<a href = "' . get_permalink($parent_area) . '">';
  // if thumbnail is needed for this list
  echo get_the_thumbnail($parent_area);
  echo get_the_title($parent_area);
  echo '</a>';
  if (is_array($child_areas)) {
    echo '<ul class="second-level">';
    foreach ($child_areas as $child_area) {
      echo '<li><a href="' . get_the_permalink($child_area) . '">' . get_the_title($child_area) . '</a></li>';
      }
    echo '</ul>';
  }
  echo '</li>';



 }
 echo '</ul>';
 ?>