gistfile1.yml

#definizione servizi 
#path/to/my_bundle/Resources/config/services.yml

services:
    my_bundle.json_handler:
            class: MyBundle\..\View\JsonViewHandler

    my_bundle.view_handler:
        parent: fos_rest.view_handler.default
        calls:
            - ['registerHandler', [ 'json', [@my_bundle.json_handler, 'createResponse'] ] ]

gistfile2.php

//classe custom handler'

namespace MyBundle\....\View;

use FOS\RestBundle\View\View,
    FOS\RestBundle\View\ViewHandler,
    Symfony\Component\HttpFoundation\Request,
    Symfony\Component\HttpFoundation\Response;

class JsonViewHandler
{
    /**
     * @param ViewHandler $viewHandler
     * @param View $view
     * @param Request $request
     * @param string $format
     *
     * @return Response
     */
    public function createResponse(ViewHandler $handler, View $view, Request $request, $format)
    {
        //difinizione della logica 
        ..
        $data = ... ;
        
        return new Response(data, 200, $view->getHeaders());
    }
}