In a large pot, combine onions, carrots, mushrooms, celery, leek (if using), garlic, parsley, thyme, bay leaves, salt, peppercorns, and water.

Bring the mixture to a boil over high heat. Once boiling, reduce heat to low and simmer, loosely covered with lid vented, for 1 hour.

After simmering, strain the stock through a fine mesh sieve, discarding the solids.

Allow the stock to cool before transferring to storage containers. Refrigerate for up to 1 week or freeze for up to 3 months.