function mytheme_preprocess_block__menu_block(&$variables) {
  if ($variables['elements']['#id'] == 'left_navigation' && !empty($variables['elements']['content']['#items'])) {
    $children = $variables['elements']['content']['#items'];
    $top_child = key($children);

    $menu_link_manager = Drupal::service('plugin.manager.menu.link');
    $menu_link_parents = $menu_link_manager->getParentIds($top_child);

    // the one we want should be the second one. the first one is the one in the main navigation across the top.
    $parent = end($menu_link_parents);
    $parent = str_replace('menu_link_content:', '' , $parent);
    $menu_content = current(Drupal::entityTypeManager()->getStorage('menu_link_content')->loadByProperties(array('uuid' => $parent)));

    $variables['top_parent'] = [
      'title' => $menu_content->getTitle(),
      'url' => $menu_content->getUrlObject()
    ];
  }
}

$link = $this->menuActiveTrail->getActiveLink();

if(!empty($link->getParent())) {
  $parent_link = Drupal::service('plugin.manager.menu.link')->createInstance($link->getParent());
  $menu_item = MenuLinkContent::load($parent_link->getMetaData()['entity_id']);
}