gistfile1.php

<?php

$this->navigation()->getPluginManager()->setAllowOverride(true)->setInvokableClass('menu', '\My\Helper');

// In Module.php
    public function getViewHelperConfig()
    {
        return array(
            // Change the Navigation View Helper plugins for ALL modules
            'initializers' => array(
                'changeNavigationHelpers' => function($helper) {
                    if ($helper instanceof \Zend\View\Helper\Navigation) {
                        $helper->getPluginManager()->setAllowOverride(true)->setInvokableClass('menu', '\My\Helper')
                    }
                },
            ),
        );
    }