One way to return a HAL entity from an Apigility RPC controller.

rpc-hal-entity.php

<?php
public function someAction()
{
    $data = doSomeWorkFetchSomeData();
    $entity = new \ZF\Hal\Entity($data, $data['id']); // or similar
    return new \ZF\ContentNegotiation\ViewModel([
        'payload' => $entity,
    ]);
}

hal links are not generated using this way. Any suggestions? Thank U.

public function categoryBySlugAction()
{
    $slug = (string) $this->params()->fromRoute('slug', '');        
    if (empty($slug)){
        throw new \Exception("A slug is necessary");
    }
    $data = $this->mapper->fetch($slug);        
    $entity = new Entity($data, $data['id']);
    return new ViewModel(array(
        'payload' => $entity,
    ));
}

getting data

public function findBySlug($slug)
{
    $qb = $this->_em->createQueryBuilder();
    $qb->select(array('c', 's', 'cp', 'cc'));
    $qb->from('Guiaville\Entity\Category', 'c');
    $qb->leftJoin('c.site', 's' );
    $qb->leftJoin('c.parent', 'cp' );
    $qb->leftJoin('c.children', 'cc' );
    $slug = $qb->expr()->literal($slug);
    $qb->where($qb->expr()->eq('c.slug', $slug));

    return $qb->getQuery()->getOneOrNullResult(Query::HYDRATE_ARRAY);
}

Incase someone else has this problem.
You can use the Hal plugin that the Rest controllers use to generate Hal entities containing links:

<?php
    public function anRpcAction()
    {
        $entity = $this->getEntityFromSomewhere();

        $route = $this->getEvent()->getRouteMatch();

        return new ViewModel([
            'payload' => $this->Hal()->createEntity($entity, $route, $route->getMatchedRouteName())
        ]);
    }

If you havn't defined a REST service for the entity you want to return, you might need to configure the metadata map by hand

'zf-hal' => [
        'metadata_map' => [
            'User' => [
                'route_name' => 'api.rest.user',
                'entity_identifier_name' => 'username',
                'route_identifier_name' => 'user_id',
                'hydrator' => 'Zend\Stdlib\Hydrator\ObjectProperty',
            ],
        ],
    ],