2 pounds mushrooms (button or a mix)
2 cups finely chopped onions
8 cups water
1 teaspoon salt
Trim and chop mushrooms finely by hand or in a food processor (do it in batches).
In a heavy stockpot, put mushrooms, onions, water and salt. Simmer uncovered, 2-3 hours. Pour stock through a large fine sieve into a large bowl, pressing hard on solids. You should have about 5 cups. Adjust either by adding water, or reducing further.
This is a great easy-to-make vegan/vegetarian stock and very flavorful. You can freeze in batches and thaw as needed for soups and sauces. The subtle flavor actually reminds me of the duck broth found in the Filipino delicacy balut.