How to access Item->ID in custom walker

I am working on building a custom Walker_Nav_Menu class for use in wp_nav_menu(). My issue is that I need to gain access to the $item->ID in start_lvl(), the same way you would in start_el().

function start_lvl( &$output, $depth, $args ) {
// You can not access $item here because it is not passed as an argument
} 

function start_el(&$output, $item, $depth, $args) {
// You can access $item here
}

I am trying to pull metadata information for each menu item using its ID. I have seen a couple other threads where people have asked similar questions but I have not found a working answer.

Any help would be much appreciated!