Put a pot of water on the fire and bring it to a boil.
In the meantime, clean the celery and cut it into chunks (keep the leaves aside and use them for other preparations).
When the water boils, dip the celery and cook it for about 10 minutes or until it is soft but still firm enough, then drain it.
Melt the butter in a pan, add the celery and cook it for a few minutes with salt and pepper, stirring it often.
At the end of cooking also add the Parmesan.