Put the dried porcini in a bowl with the water and soak for at least 15 minutes.
Peel and finely chop the onion. Peel and cut the celery into ¼-inch dice. Peel the potatoes and cut into ½-inch dice. Put the potatoes in a bowl and cover with cold water to prevent them from discoloring.
Put the onion, celery, and olive oil in a soup pot and place over m