Krupnik (Polish Mushroom Barley Soup)

1 -1 1/2 ounce dried porcini mushrooms (cepes)
13 cups water or 13 cups stock (or more)
2 small carrots (optional)
2 small turnips (optional)
2 small onions
2 medium potatoes (optional)
2 stalks celery & leaves (optional)
3/4 cup pearl barley
sour cream (optional)
Soak the cepes in a little water for 15 minutes, until they soften.
Chop all the vegetables finely in 2 batches in the food processor and put them in a pan with the barley and all the water (including the mushroom-soaking water).
Chop the softened mushrooms in the food processor, and add them to the pot.
Bring to the boil, remove the scum, season with plenty of salt and pepper, and simmer for 1 hour, or until the barley is very soft and plump.
The barley will thicken the soup.
Add water if necessary, to thin it.
Serve with sour cream, especially if the soup was made with water instead of broth.