I want to update my existing API (created with FOSRest).

I have lots of routes that return custom JSON objects, different from my entities.

For example, I have an Offer entity

<?php
// api/src/Entity/Offer.php

namespace App\Entity;

use ApiPlatform\Core\Annotation\ApiResource;
use Doctrine\ORM\Mapping as ORM;
use Symfony\Component\Validator\Constraints as Assert;

/**
 * An offer from my shop - this description will be automatically extracted form the PHPDoc to document the API.
 *
 * @ApiResource(iri="http://schema.org/Offer")
 * @ORM\Entity
 */
class Offer
{
    /**
     * @ORM\Column(type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    public $id;

    /**
     * @ORM\Column(type="text")
     */
    public $description;

    /**
     * @ORM\Column(type="float")
     * @Assert\NotBlank
     * @Assert\Range(min=0, minMessage="The price must be superior to 0.")
     * @Assert\Type(type="float")
     */
    public $price;

    /**
     * @ORM\ManyToOne(targetEntity="Product", inversedBy="offers")
     */
    public $product;
}

And I want to return a JSON object based on that product entity but with custom fields.

Is it really mandatory to create a new resource just for that one GET method or is there a best practice to do that?

There is multiple ways to add custom data :

(simplest) Add a custom entity method

class Offer
{
    public function getSomethingCustomized() 
    {
        return 'something_customized';
    }
}

You might need to use the serialization groups too.

Decorate the serializer

If your custom data needs to be pulled from outside the entity : Here is the documentation

It overrides the default Serializer service (called before sending the response), and allow you to add extra data (from wherever you want) to each request.