Trim, halve, and rinse the leeks, then cut into chunks. In a stockpot, combine the leeks, onions, carrots, celery, parsley, mushrooms, and peppercorns. Add 5 qt (5 l) water and bring to a boil over high heat. Reduce the heat to medium-low, cover partially, and simmer until the vegetables are very soft and the flavors have blended, about 1 hour. Season to taste with salt. Pour the contents of the pot through a fine-mesh sieve into a large bowl. Press down on the solids to extract all the liquid, and discard the solids.