I guess you extended SyliusWebBundle. You can extend Sylius\Bundle\WebBundle\Menu\BackendMenuBuilder
to Tom\Bundle\WebBundle\Menu\BackendMenuBuilder
and change parameter
<parameter key="sylius.menu_builder.backend.class">Sylius\Bundle\WebBundle\Menu\BackendMenuBuilder</parameter>
to
<parameter key="sylius.menu_builder.backend.class">Tom\Bundle\WebBundle\Menu\BackendMenuBuilder</parameter>