How to Make It
Ideally, you should soak the dried mushrooms in a quart of warm water overnight to rehydrate them. The soaking liquid is the soup stock, so be careful not to throw it away.
When mushrooms are soft, remove from water, squeezing most of the liquid back into the bowl. Trim away hard woody parts of the stems. Coarsely chop mushrooms, and set aside. Pour liquid through a cheesecloth-lined sieve over a bowl—stop when a tablespoon or so of liquid remains, and discard it along with the grit. Combine strained liquid with water until you have 10 cups.
Heat the oil in a large soup pot over medium-high. Add the fresh mushrooms, onion, celery, and garlic, and cook, stirring occasionally, until mushrooms give up their water, about 10 minutes. Add thyme, dill, white pepper, and cayenne, and keep stirring. Sprinkle flour over mushroom mixture, and stir constantly for a few minutes until flour begins to stick to the pot. Add porcini mushrooms, the 10 cups of mushroom liquid, and bay leaf. Stir while scraping to deglaze bottom of pot. Cook 15 minutes, stirring as mixture thickens.
Reduce heat to a simmer, stir in potatoes, and cook until meltingly tender, another 45 minutes. Season with vinegar, salt, and hot sauce to taste. Simmer a few more minutes. Remove bay leaf. Serve family style in a tureen; garnish with fresh dill.