Wash the Parmesan rinds under running water (read the instructions I wrote in the Ingredients section).
Wash celery and carrot and set aside.
In a large, high-sided pan, place Parmesan rinds, celery, carrot, leek, bay leaves and salt.
Cover with cold water and bring to a boil.
Low the heat and cook on a small stove for two hours. Cover the pot almost entirely with a lid.
Turn off, remove vegetables and rinds from the broth, and strain using a sieve.
Pour into a bottle or glass jar and store in the refrigerator for about a week. You can also freeze for up to a month. In this case, remember not to fill the jar to the rim because as it freezes, the liquid expands.